SEARCH

SEARCH BY CITATION

Keywords:

  • Cell division;
  • Regulation;
  • Fts protein;
  • Transcriptional organization;
  • Gene expression;
  • Escherichia coli

Abstract

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Cell growth and division: the E. coli life cycle
  5. 3Transcriptional control of the mra cluster: a plethora of promoters
  6. 4Cranking the speed: gearboxes and growth rate independence
  7. 5Beyond mRNA: processing and post-transcriptional controls
  8. 6Protein regulators of mra transcription
  9. 7Concluding remarks
  10. References

Duplication of the Escherichia coli bacterial cell culminates in the formation of a division septum that splits the progenitor cell into two identical daughter cells. Invagination of the cell envelope is brought about by the co-ordinated interplay of a family of septation-specific proteins that act locally at mid-cell at a specific time in the cell cycle. The majority of the genes known to be required for septum formation are found within the large mra cluster located at 2 min on the E. coli genetic map (nucleotides 89 552–107 474). Examination of the controls exerted on the mra operon shows that E. coli uses an extraordinary range of strategies to co-ordinate the expression of the cell division genes with respect to each other and to the cell cycle.


1Introduction

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Cell growth and division: the E. coli life cycle
  5. 3Transcriptional control of the mra cluster: a plethora of promoters
  6. 4Cranking the speed: gearboxes and growth rate independence
  7. 5Beyond mRNA: processing and post-transcriptional controls
  8. 6Protein regulators of mra transcription
  9. 7Concluding remarks
  10. References

The speed and efficiency with which bacterial cells can replicate is unequalled. During rapid growth in rich medium, Escherichia coli cells can duplicate in a little over 20 min, which demands the seamless integration of general metabolism with the dedicated biochemical and genetic pathways that drive cell growth, chromosome replication and ultimately, cell division. Cells in balanced growth rarely get this wrong; it is exceptional to find a DNA-less cell, or one which is of greater or lesser mass than the others, testament to the exquisite proficiency of the control networks that co-ordinate these events. While we are still some way from understanding the intracellular signals that transduce the physiological state of the cell into cell growth, the global networks that harmonise patterns of gene expression in response to changing environmental conditions such as starvation (σS) or heat shock (σ32, σE) are increasingly well understood. A number of recent molecular studies have also shed light on the regulatory networks that ensure smooth progression through the growth and division cycle. This review will examine the diverse regulatory strategies, including a growth-rate-dependent mechanism, antisense RNAs and quorum-sensing regulatory proteins, that control the synthesis and activity of the primary division genes located in the 2-minute mra cluster, nucleotides 89 552–107 474.

2Cell growth and division: the E. coli life cycle

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Cell growth and division: the E. coli life cycle
  5. 3Transcriptional control of the mra cluster: a plethora of promoters
  6. 4Cranking the speed: gearboxes and growth rate independence
  7. 5Beyond mRNA: processing and post-transcriptional controls
  8. 6Protein regulators of mra transcription
  9. 7Concluding remarks
  10. References

Our current understanding of the E. coli cell cycle has been greatly aided by the study of temperature-sensitive growth and division mutants, while recent advances in fluorescent microscopy have provided insights into the dynamic process of development in living bacterial cells. The life cycle of E. coli appears straightforward. The bacterium grows in length with little change in diameter, until it reaches a critical size that is twice its original length. Cell division is then initiated at mid-cell with the formation of a contractile ring, comprised largely of the tubulin-like protein, FtsZ. Evidence suggests that initiation of the Z-ring is prompted by the activation of a single site at mid-cell in response to an as yet uncharacterised cell cycle signal. Although the factors that regulate the timing of Z-ring assembly are not yet well understood, a series of elegant molecular studies on the Min proteins has gone a considerable way towards describing the mechanism by which topological specificity in the placement of the ring may be achieved (reviewed in [1]). The Z-ring provides a framework onto which at least eight other septation proteins, including ZipA, FtsA, FtsQ, FtsL, FtsK, FtsI, FtsN, and FtsW, are recruited.

Control of the intracellular concentration of the septation-specific proteins is of pivotal importance for cell division. Even small fluctuations in the levels of the essential cell division proteins can severely disrupt cell growth [2–4]. Mutants that either cannot make, or make a thermolabile form of any of the individual Fts proteins, are blocked in division and grow as smooth or partially constricted filaments depending on the stage at which the gene product acts [5]. In contrast, the modest overproduction of FtsZ protein, which is rate-limiting for division, induces extra divisions as evidenced by the production of minicells at cell poles; while still higher levels of FtsZ block cell division. Indeed, the molar ratio of FtsA to FtsZ is critical for correct cell growth, since elevating the level of one protein at the expense of the other results in filamentation, an effect that may be suppressed by the co-ordinate overexpression of the other [2,3].

Under normal conditions therefore, the concentration of the division proteins must be maintained within defined limits to avoid the potentially damaging effects of unbalanced protein synthesis. In view of this, it is not surprising that the cell division genes are subject to complex regulation. The majority of the genes that are essential and specific for cell division are located in the 2-min mra cluster. Expression of these genes is regulated by an elaborate arrangement of interwoven controls that ensures the division proteins are synthesised in appropriate amounts during the cell cycle.

3Transcriptional control of the mra cluster: a plethora of promoters

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Cell growth and division: the E. coli life cycle
  5. 3Transcriptional control of the mra cluster: a plethora of promoters
  6. 4Cranking the speed: gearboxes and growth rate independence
  7. 5Beyond mRNA: processing and post-transcriptional controls
  8. 6Protein regulators of mra transcription
  9. 7Concluding remarks
  10. References

Even cursory examination of the arrangement of genes within the mra cluster attests to their close association and unconventional arrangement. Sixteen genes encoding proteins involved in cell division and/or envelope biosynthesis are crowded together in a contiguous run spanning 17.9 kb. The genes are all read in the same direction, with many of the ORFs overlapping or separated by only very short gaps. The regulatory signals for individual genes are routinely found in the neighbouring upstream gene (Fig. 1). This unusual genetic arrangement yields a set of overlapping mRNAs initiated from a series of upstream promoters. Transcription of ftsZ, which lies at the distal end of the cluster, is directed from multiple promoters, some of which lie many hundreds of nucleotides upstream of it. At least six ftsZ promoters lie within the three genes immediately adjacent to it. Of these, two promoters lie within ddlB (ftsQ2p1p) and transcribe the downstream genes, ftsQ, ftsA, ftsZ (and finally envA which precedes the single strong terminator at the end of the cluster). A single promoter within ftsQ (ftsAp) transcribes ftsA and ftsZ, while the three promoters within ftsA (ftsZ4p3p2p) transcribe ftsZ[6–9]. EnvA has its own promoter located in the intergenic space between it and ftsZ suggesting that, while ftsQ, ftsA and ftsZ may be co-ordinately expressed, transcription of envA is likely to be distinct from that of its upstream neighbours. The absence of transcriptional terminators within the cluster potentially allows transcripts initiated at the mra promoter at its proximal end to transcribe the ftsZ and envA genes some 17 kb downstream. In keeping with this observation, Dai and Lutkenhaus [10] reported that sequences at the 5′ end of the cluster were required for full ftsZ expression. They found that the prophage λ16-2, which carries a 6-kb fragment of DNA extending from the middle of ftsW to just beyond envA, was unable to complement a null allele of ftsZ, but that the more extensive chromosomal insert carried on F′104 could. More recently, Flärdh et al. [11] have used a φ(ftsZlacZ) fusion introduced into the chromosomal locus to monitor transcription reaching ftsZ from upstream promoters. They were able to show that the insertion of strong transcriptional terminators into ddlB (ddlB::Ω) reduced transcription of ftsZ by two-thirds, supporting the tenet that distant upstream elements are required for its full expression. The nature of these activating upstream sequences is not yet clear, but two recent studies support the possibility of there being some contribution from the mra promoter. Hara et al. [12] have shown that Pmra is necessary for full expression of the genes from mraZ to ftsW, while a subsequent study by Mengin-Lecreulx et al. [13] showed that substitution of Pmra by Plac causes a concomitant 30% reduction in the levels of FtsZ protein. Several promoter-like sequences have been described for both the ftsImurD and the orfCftsL regions [13–15] which, if shown to be active, may also contribute to the expression of the downstream genes.

image

Figure 1. The chromosomal arrangement of the 2-min mra cluster is shown on the upper diagram. The direction of transcription is indicated, as is the position of the single transcriptional terminator (T) that lies after the envA gene. The fine structure of the ddlB–ftsZ region is detailed below. Open arrowheads indicate promoters, their designation is shown underneath, while the antisense transcript (stfZ) is shown by a filled arrowhead. DnaA binding sites are represented by filled circles and the inverted repeat regulatory sequences within ftsQ, by inverted arrows. The letter E indicates the positions of two RNase E processing sites. The method by which the promoters internal to the ddlB–ftsZ region are regulated is summarised in the table. A ‘+’ symbol indicates the promoter is activated under the given conditions, a ‘−’ that it is insensitive to them. When more than one promoter may be subject to regulation, brackets denote the promoters included. The mraZ–envA region is drawn to scale.

Download figure to PowerPoint

Gene fusions have been used extensively to determine the contribution of the promoters within the ddlBftsZ locus (in the absence of the upstream sequences) to transcription of ftsZ[8,9,16]. By constructing fusions between the ddlBftsQAZ sequence and lacZ on a λ vector, it has been shown that approximately 45% of transcription from within this region is initiated from ftsQ2p1p, the two promoters that lie within ddlB, while ftsZ4p3p contribute a further 37%. The remaining activity is accounted for from ftsAp, 12% and ftsZ2p, 5%[8]. Several of these promoters have been shown to be growth-rate-sensitive, and variations in mRNA levels between studies may feasibly be caused by culture conditions generating a variety of growth rates.

A distinctly different transcriptional pattern emerges when the DNA sequence upstream of ddlB is included in the analysis and transcription reaching ftsZ from these upstream sequences is monitored from chromosomal fusions to the native ftsZ gene rather than from prophage integrants or from plasmids. Using this system it becomes apparent that during rapid growth the bulk of ftsZ transcripts (>66%) are initiated from sequences upstream of ddlB[11]. The ftsQ2p1p promoters contribute only a fraction of their plasmid-encoded value at 15% and ftsZ4p3p are reduced to 12%. A minor contribution is made from ftsAp and ftsZ2p whose levels are also reduced slightly to 4% and 2%, respectively. Despite the change in emphasis on the importance of the upstream promoters for transcription of ftsZ, the multiple controls exerted over the ftsZ proximal promoters suggest that they are of significant importance in determining the final levels of the cell division proteins. They may modulate synthesis of the proteins in response to particular intracellular signals or environmental conditions or play a role in establishing the correct relative ratio of division proteins. How might regulation of these promoters be achieved?

4Cranking the speed: gearboxes and growth rate independence

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Cell growth and division: the E. coli life cycle
  5. 3Transcriptional control of the mra cluster: a plethora of promoters
  6. 4Cranking the speed: gearboxes and growth rate independence
  7. 5Beyond mRNA: processing and post-transcriptional controls
  8. 6Protein regulators of mra transcription
  9. 7Concluding remarks
  10. References

The rate of transcription of ftsZ varies with cell growth so that it increases as the generation time lengthens [6,17,18]. Since average cell size is larger in fast-growing cells, the overall effect of this growth-rate-regulated control is to maintain constant the in vivo levels of FtsZ protein per cell cycle. Of the six promoters that contribute to ftsZ transcription from within the ddlBftsZ region, ftsQp1, ftsZ4p and 3p are inversely growth-rate-dependent and increase in activity on entry into stationary phase [8,17,18]. The regulation of ftsQ2p and 1p is of particular interest, since these companion promoters account for almost half of ftsZ transcription. When cells are growing quickly ftsQ2p is strongly induced, while ftsQ1p is induced as the growth rate declines until it becomes predominant at low growth rates [6].

FtsQ2p is regulated by a quorum-sensing mechanism effected through SdiA, a member of the LuxR family of transcriptional activators ([19], and see below) while ftsQ1p, which belongs to a family of ‘gearbox’ promoters, is positively regulated by the stationary growth phase sigma factor σS[20,21]. The growth rate dependence of an ftsQp1::lacZ fusion is abolished in a strain in which the rpoS gene is insertionally inactivated. The effect of the gearbox is to ensure the production of constant amounts of product, in this case FtsQ, FtsA and to some degree FtsZ, per cell at any growth rate [22]. Gearbox promoters share sequence homology in their promoter regions; ftsQ1p has strong sequence homology with the stationary-phase-activated bolA promoter (bolA expression causes cells to become smaller and more rounded as they move into stationary phase). The promoters include a short AT-rich region (a UP element) thought to be activated by the carboxy-terminal domain of the α-subunit of RNA polymerase [23,24]. The housekeeping promoter ftsQ2p is only transcribed by EσD, whereas ftsQ1p is recognised by RNA polymerase sigma factors σD and σS, with EσS being favoured on entry into stationary phase [25]. When the relative contributions of ftsZ2p3p4p are analysed at different growth rates, 4p, 3p and 2p also show an inverse growth rate dependence, as well as moderate induction in stationary phase [17,18]. In contrast with ftsQ1p, in vitro studies show that ftsZ2p3p4p are preferentially transcribed by EσD rather than by EσS, so although they exhibit a growth-phase-dependent mode of expression, it must be achieved through an uncharacterised mechanism that is distinct from the RpoS-dependent gearbox system.

There have been several reports suggesting that transcription of ftsZ is periodic in the cell cycle, with transcription taking place just before cell division [17,26–28]. Garrido et al. [28] used RT-PCR to show that the abundance of ftsZ mRNA oscillates periodically during the cell cycle, becoming maximal coincident with the initiation of DNA replication. A second study, also using cells synchronised by membrane elution, showed that ftsZ expression is maximal around the middle of the cell cycle and minimal at the time of cell division [29]. It had been suggested that DnaA protein might provide a link between the control of initiation and cell division, although cell division is not triggered by chromosome replication per se [30]. Transcription of ftsZ from an ftsAp–ftsZ4p3p–lacZ fusion on a λ phage was shown to increase at the non-permissive temperature in a dnaA46 host, potentially mediated by the three DnaA boxes located within the ftsQ and ftsA genes [31]. However, re-examination of the role of DnaA in the regulation of ftsZ concluded that the effects described earlier were likely to result from physiological changes and were not specific to DnaA [28,32]. In a strain in which ftsZ is separated from its natural chromosomal regulatory signals and is instead inducible from the tac promoter by the addition of exogenous IPTG, ftsZ loses its periodic expression and is expressed continuously throughout the cell cycle [4,28]. Surprisingly, cells induced to overproduce FtsZ by 40% above wild-type levels are unaffected in cell growth, suggesting that additional factors ensure the correct timing of cell division. How the periodic expression of division genes is achieved remains unclear.

5Beyond mRNA: processing and post-transcriptional controls

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Cell growth and division: the E. coli life cycle
  5. 3Transcriptional control of the mra cluster: a plethora of promoters
  6. 4Cranking the speed: gearboxes and growth rate independence
  7. 5Beyond mRNA: processing and post-transcriptional controls
  8. 6Protein regulators of mra transcription
  9. 7Concluding remarks
  10. References

The Fts proteins are present in widely varying amounts, despite the fact that the genes are all members of the same transcriptional unit: e.g. the ratios of the proteins are in the order of FtsZ (5000–20 000 [33,34]):FtsA (50–200 [35]):PBP3 (100 [36,37]):FtsQ (25 [38]):FtsL (20–40 [14]). In part, this differential expression is due to the different numbers and strengths of the internal promoters described above, but it is also controlled through processing (and decay) of the polycistronic transcripts and in the relative rates of translation of the division proteins.

A potentially important element in the regulation of gene expression is the stability of mRNA, and control of the rate of mRNA decay is one means by which cells can reduce the rate of biosynthesis of proteins during times of slow growth [39]. The decay rate of transcripts does vary, thus contributing to the differential expression of ftsZ:ftsA. Two major in vivo RNase E processing sites have been identified [9], one within the ftsA coding sequence (this was originally designated ftsZp1 [6] but actually corresponds to the RNase E1/E2 cleavage sites) and a second in the ftsA–Z intergenic region. Cleavage by RNase E destabilises ftsA and possibly ftsQ mRNA, altering the ratio of ftsA to ftsZ mRNA approximately five-fold, and thereby contributing to the different levels of expression of ftsA to ftsZ observed. It has also been reported that cells lacking the host factor-I (HF-I, which is encoded by the hfq gene at 95 min on the E. coli chromosome) produce minicells at high frequency in stationary phase and in poor medium [40]. The Hfq protein is thought to regulate ompA mRNA stability by modulating RNase E-dependent cleavage of its mRNA and it has been suggested that Hfq may regulate the stability of ftsZ mRNA in a similar manner; hfq::cat mutants exhibit two- to three-fold higher FtsZ protein levels than are observed in exponential phase cells. The increase in FtsZ protein reflects an increase in ftsZ mRNA abundance and suggests that Hfq acts as a negative regulator of ftsZ in cell division. In addition to the transcriptional controls discussed above, there are also differences in translational efficiencies of ftsQ, ftsA and ftsZ from the polycistronic transcripts. FtsQ and FtsA messages are translated at low level (FtsQ<FtsA) from separate ribosome binding sites (RBS) on a common mRNA while FtsZ is translated at a higher level [41]. The low rates of synthesis of FtsQ and FtsA result from the low binding affinity of RNA polymerase to their ribosome binding sequences. Replacing the upstream sequences with the RBS and initial few codons of an efficiently translated gene, gene 10 of phage T7, resulted in the production of up to 25-fold more protein [41].

A short coding gap exists between the stop codon of ftsA and the start codon of ftsZ. The presence of this region in high copy blocks cell division at elevated temperature [42]. It has been suggested that this may result from the production of an antisense RNA (stfZ), or even a small inhibitor protein from the sequence, although the molecular basis of the inhibition remains unclear. There is a precedent for the action of an antisense RNA in potentiating production of FtsZ protein, since the mRNA product of the dicF gene inhibits the synthesis of FtsZ in a similar fashion [43]. DicF, thought to be part of the lamboid phage relic Kim, encodes a 53-nucleotide mRNA that blocks cell division by binding to, and inhibiting translation of, ftsZ mRNA. The function of the dicA operon, of which dicF is a part, may have been to stop cell division during lytic multiplication of the phage; it has no discernible physiological role in the cell under normal growth conditions [44]. If this is the case, then the partial complementarity of dicF to the RBS region of ftsZ mRNA may mimic the action of a natural division inhibitor such as stfZ. Since transcription of an inhibitor like stfZ would specifically inhibit FtsZ production, controlling its transcription might provide a route to control the timing of cell division. However, as with some of the earlier control mechanisms, the main problem is in identifying “how the regulators are themselves regulated in relation to the cell's periodic needs for division proteins”[45].

6Protein regulators of mra transcription

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Cell growth and division: the E. coli life cycle
  5. 3Transcriptional control of the mra cluster: a plethora of promoters
  6. 4Cranking the speed: gearboxes and growth rate independence
  7. 5Beyond mRNA: processing and post-transcriptional controls
  8. 6Protein regulators of mra transcription
  9. 7Concluding remarks
  10. References

There are conflicting reports on the autoregulation of genes at the mra locus. Transcription from the promoters within ftsQ and ftsA has been shown to be derepressed in cells lacking FtsA, leading to the suggestion that FtsA may act as a direct repressor of one or both of these promoters [17], although this was later challenged by Robin et al. [27]. Dai and Lutkenhaus [10] established that ftsZ is not autoregulated within the ftsA–Z intergenic region, an observation that has been extended to include the entire mra cluster [43]. Transcription from the ftsAp promoter within ftsQ has been shown to be influenced by upstream sequences that form an inverted repeat [7]; however, while these sequences may constitute a target for a protein regulator, no specific binding proteins interacting with them have yet been described. The product of the unlinked sdiA gene up-regulates the ftsQ2p promoter and its overexpression results in a small increase in the production of FtsZ protein [46]. However, sdiA itself is down-regulated by a signalling molecule that is released into the medium during E. coli cell growth, suggesting that cell-to-cell signalling may play a role in regulating division gene expression [47]. SdiA is not essential, because deletion of the gene has no obvious deleterious effect on cell growth or division. The cellular alarmone guanosine tetraphosphate (ppGpp) has also been implicated as a possible transcriptional effector of ftsZ expression during the cell cycle (reviewed in [48]). Elevated levels of ppGpp suppress the filamentation of an ftsZ84 strain at high temperature by increasing cellular FtsZ levels. It has been suggested that the down-regulation of upstream promoters by ppGpp may allow a concomitant increase in transcription from occluded promoters lying closer to ftsZ[49]. However, the mechanism may be rather more complex since Navarro et al. [50] were unable to discern any direct effect of ppGpp level on any of the ftsQAZ promoters.

The product of another unlinked gene, mreB, represses transcription of ftsI[51]. In the absence of this regulator, cells become spherical as a result of overproduction of PBP3 and possibly the genes adjacent to ftsI. Evidence has also been presented to show that transcription from an ftsQ–ftsA–lacZ fusion is induced five- to six-fold by overproduction of the rcsB gene product [52]. The rcsB gene was identified as a suppressor of the ftsZ84 division mutant, the suppression being effected by increased transcription from ftsAp to an unknown cellular stimulus [53]. RcsB is part of a two-component system that is involved in the activation of capsular synthesis and the biosynthesis of colanic acid, the connection between these pathways and cell division remains to be elucidated.

7Concluding remarks

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Cell growth and division: the E. coli life cycle
  5. 3Transcriptional control of the mra cluster: a plethora of promoters
  6. 4Cranking the speed: gearboxes and growth rate independence
  7. 5Beyond mRNA: processing and post-transcriptional controls
  8. 6Protein regulators of mra transcription
  9. 7Concluding remarks
  10. References

The cell division proteins are required exclusively for the biosynthesis of the septum, and as such are required at only very specific times in the cell cycle. The elaborate regulation of these proteins has not yet been fully defined and we are still some way from understanding how their activities are controlled within the cycle. Pre-existing proteins may be activated in response to an as yet uncharacterised cellular effector; or alternatively, transcription (and/or translation) of the division genes may be periodic. Clearly, a complex regulatory network co-ordinates expression of the division genes within the mra operon, but it is not entirely apparent why this should be so. A simple reason might be that some redundancy has evolved within the regulatory mechanisms and this provides a buffer to protect what is an essential cellular process from the deleterious effects of mutation. Alternatively, they may provide a variety of targets that respond to regulation by intra- or extracellular signals, allowing the modulation of cell growth in response to other cellular processes. It seems likely that the coming decade will uncover yet more unanticipated aspects governing their control. One of the most exciting challenges will be to develop methods to monitor these events in real time, as integral aspects of the dynamic processes of cell growth and division.

References

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Cell growth and division: the E. coli life cycle
  5. 3Transcriptional control of the mra cluster: a plethora of promoters
  6. 4Cranking the speed: gearboxes and growth rate independence
  7. 5Beyond mRNA: processing and post-transcriptional controls
  8. 6Protein regulators of mra transcription
  9. 7Concluding remarks
  10. References
  • [1]
    Bramhill, D. (1997) Bacterial cell division. Annu. Rev. Cell Dev. Biol. 13, 395424.
  • [2]
    Dai, K., Lutkenhaus, J. (1992) The proper ratio of FtsZ to FtsA is required for cell division to occur in Escherichia coli. J. Bacteriol. 174, 61456151.
  • [3]
    Dewar, S.J., Begg, K.J., Donachie, W.D. (1992) Inhibition of cell division initiation by an imbalance in the ratio of FtsA to FtsZ. J. Bacteriol. 174, 63146316.
  • [4]
    Palacios, P., Vicente, M., Sanchez, M. (1996) Dependency of Escherichia coli cell-division size, and independency of nucleoid segregation on the mode and level of ftsZ expression. Mol. Microbiol. 20, 10931098.
  • [5]
    Begg, K.J., Donachie, W.D. (1985) Cell shape and division in Escherichia coli: experiments with shape and division mutants. J. Bacteriol. 163, 615622.
  • [6]
    Aldea, M., Garrido, T., Pla, J., Vicente, M. (1990) Division genes in Escherichia coli are expressed coordinately to cell septum requirements by gearbox promoters. EMBO J. 9, 37873794.
  • [7]
    Dewar, S.J., Donachie, W.D. (1990) Regulation of expression of the ftsA cell-division gene by sequences in upstream genes. J. Bacteriol. 172, 66116614.
  • [8]
    Flärdh, K., Garrido, T., Vicente, M. (1997) Contribution of individual promoters in the ddlB-ftsZ region to the transcription of the essential cell-division gene ftsZ in Escherichia coli. Mol. Microbiol. 24, 927936.
  • [9]
    Cam, K., Rome, G., Krisch, H.M., Bouché, J.P. (1996) RNase E processing of essential cell division genes mRNA in Escherichia coli. Nucleic Acids Res. 24, 30653070.
  • [10]
    Dai, K., Lutkenhaus, J. (1991) ftsZ is an essential cell division gene in Escherichia coli. J. Bacteriol. 173, 35003506.
  • [11]
    Flärdh, K., Palacios, P., Vicente, M. (1998) Cell division genes ftsQAZ in Escherichia coli require distant cis-acting signals upstream of ddlB for full expression. Mol. Microbiol. 30, 305315.
  • [12]
    Hara, H., Yasuda, S., Horiuchi, K., Park, J.T. (1997) A promoter for the first nine genes of the Escherichia coli mra cluster of cell division and cell envelope biosynthesis genes, including ftsI and ftsW. J. Bacteriol. 179, 58025811.
  • [13]
    Mengin-Lecreulx, D., Ayala, J., Bouhss, A., Van Heijenoort, J., Parquet, C., Hara, H. (1998) Contribution of the Pmra promoter to expression of genes in the Escherichia coli mra cluster of cell envelope biosynthesis and cell division genes. J. Bacteriol. 180, 44064412.
  • [14]
    Guzman, L.M., Barondess, J.J., Beckwith, J. (1992) FtsL, an essential cytoplasmic membrane protein involved in cell division in Escherichia coli. J. Bacteriol. 174, 77167728.
  • [15]
    Ishino, F., Jung, H.K., Ikeda, M., Doi, M., Wachi, M., Matsuhashi, M. (1989) New mutations fts-36, lts-33, and ftsW clustered in the mra region of the Escherichia coli chromosome induce thermosensitive cell growth and division. J. Bacteriol. 171, 55235530.
  • [16]
    Yi, Q-M., Rockenbach, S., Ward, J.E., Lutkenhaus, J. (1985) Structure and expression of the cell-division genes ftsQ, ftsA and ftsZ. J. Mol. Biol. 184, 399412.
  • [17]
    Dewar, S.J., Kagan-Zur, V., Begg, K.J., Donachie, W.D. (1989) Transcriptional regulation of cell division genes in Escherichia coli. Mol. Microbiol. 3, 13711377.
  • [18]
    Smith, R.W.P., Masters, M., Donachie, W.D. (1993) Cell division and transcription of ftsZ. J. Bacteriol. 175, 27882791.
  • [19]
    Wang, X., de Boer, P.A.J., Rothfield, L.I. (1991) A factor that positively regulates cell division by activating transcription of the major cluster of cell division genes in Escherichia coli. EMBO J. 10, 33633372.
  • [20]
    Lange, R., Hengge-Aronis, R. (1991) Growth phase regulated expression of bolA and morphology of stationary-phase Escherichia coli cells are controlled by the novel sigma factor σS. J. Bacteriol. 173, 44744481.
  • [21]
    Sitnikov, D., Schineller, J.B., Baldwin, T.O. (1996) Control of cell division in Escherichia coli: regulation of transcription of ftsQA involves both rpoS and SdiA-mediated autoinduction. Proc. Natl. Acad. Sci. USA 93, 336341.
  • [22]
    Vicente, M., Kushner, S.R., Garrido, T., Aldea, M. (1991) The role of the ‘gearbox’ in the transcription of essential genes. Mol. Microbiol. 5, 20852091.
  • [23]
    Ross, W., Gosink, K.K., Salomon, J., Igarashi, K., Sou, C., Ishihama, A., Severinov, K., Gourse, R.L. (1993) A third recognition element in bacterial promoters: DNA binding by the α-subunit of RNA polymerase. Science 262, 14071413.
  • [24]
    Blatter, E.E., Ross, W., Tang, H., Gourse, R.L., Ebright, R.H. (1994) Domain organisation of RNA polymerase α-subunit: C-terminal 85 amino acids constitute a domain capable of dimerisation and DNA bending. Cell 78, 889896.
  • [25]
    Ballesteros, M., Kusano, S., Ishihama, A., Vicente, M. (1998) The ftsQ1p gearbox promoter of Escherichia coli is a major sigma S-dependent promoter in the ddlB-ftsA region. Mol. Microbiol. 30, 419430.
  • [26]
    Donachie, W.D., Sullivan, N.F., Kenan, D.J., Derbyshire, S.V., Begg, K.J. and Kagan-Zur, V. (1983) Genes and cell division in E. coli. In: Progress in Cell Cycle Controls (Chaloupka, J., Lotyk, A. and Streiblova, E., Eds.), pp.28–33. Czechoslovak Academy of Science, Prague.
  • [27]
    Robin, A., Joseleau-Petit, D., D'Ari, R. (1990) Transcription of the ftsZ gene and cell division in Escherichia coli. J. Bacteriol. 172, 13921399.
  • [28]
    Garrido, T., Sanchez, M., Palacios, P., Aldea, M., Vicente, M. (1993) Transcription of ftsZ oscillates during the cell cycle of Escherichia coli. EMBO J. 12, 39573965.
  • [29]
    Zhou, P., Helmstetter, C.E. (1994) Relationship between ftsZ gene expression and chromosome replication in Escherichia coli. J. Bacteriol. 176, 61006106.
  • [30]
    Bernander, R., Nordström, K. (1990) Chromosome replication does not trigger cell division in Escherichia coli. Cell 60, 365374.
  • [31]
    Masters, M., Paterson, T., Popplewell, A.G., Owen-Hughes, T., Pringle, J.H., Begg, K.J. (1989) The effect of DnaA protein levels and the rate of initiation at oriC on transcription originating in the ftsQ and ftsA genes – in vivo experiments. Mol. Gen. Genet. 216, 475483.
  • [32]
    Smith, R.W.P., McAteer, S., Masters, M. (1996) The coupling between ftsZ transcription and initiation of DNA replication is not mediated by the DnaA-boxes upstream of ftsZ or by DnaA. Mol. Microbiol. 21, 361372.
  • [33]
    Bi, E., Lutkenhaus, J. (1991) FtsZ ring structure associated with division in Escherichia coli. Nature 354, 161164.
  • [34]
    Pla, J., Sanchez, M., Palacios, P., Vicente, M., Aldea, M. (1991) Preferential cytoplasmic location of ftsZ, a protein essential for Escherichia coli septation. Mol. Microbiol. 5, 16811686.
  • [35]
    Wang, H.C., Gayda, R.C. (1992) Quantitative determination of FtsA at different growth-rates in Escherichia coli using monoclonal antibodies. Mol. Microbiol. 6, 25172524.
  • [36]
    Spratt, B.G. (1977) Temperature-sensitive cell division mutants of Escherichia coli with thermolabile penicillin-binding proteins. J. Bacteriol. 131, 293305.
  • [37]
    Dougherty, T.J., Kennedy, K., Kessler, R.E., Pucci, M.J. (1996) Direct quantitation of the number of individual penicillin-binding proteins per cell in Escherichia coli. J. Bacteriol. 178, 61106115.
  • [38]
    Carson, M.J., Barondess, J., Beckwith, J. (1991) The FtsQ protein of Escherichia coli– membrane topology, abundance, and cell division phenotypes due to overproduction and insertion mutations. J. Bacteriol. 173, 21872195.
  • [39]
    Nilsson, G., Belasco, J.G., Cohen, S.N., von Gabain, A. (1984) Growth-rate dependent regulation of messenger RNA stability in Escherichia coli. Nature 312, 7577.
  • [40]
    Takada, A., Wachi, M., Nagai, K. (1999) Negative regulatory role of the Escherichia coli hfq gene in cell division. Biochem. Biophys. Res. Commun. 266, 579583.
  • [41]
    Mukherjee, A., Donachie, W.D. (1990) Differential translation of cell division proteins. J. Bacteriol. 172, 61066111.
  • [42]
    Dewar, S.J., Donachie, W.D. (1993) Antisense transcription of the ftsZ-ftsA gene junction inhibits cell division in Escherichia coli. J. Bacteriol. 175, 70977101.
  • [43]
    Tétart, F., Bouché, J.P. (1992) Regulation of the expression of the cell-cycle gene ftsZ by DicF antisense RNA. Division does not require a fixed number of FtsZ molecules. Mol. Microbiol. 6, 615620.
  • [44]
    Bouché, F., Bouché, J.P. (1989) Genetic evidence that DicF, a second division inhibitor encoded by the Escherichia coli dicB operon, is probably RNA. Mol. Microbiol. 3, 991994.
  • [45]
    Donachie, W.D. (1997) The cell cycle of Escherichia coli: a growing mystery. Not published for general release.
  • [46]
    Wang, X., de Boer, P.A.J., Rothfield, L.I. (1991) A factor that positively regulates cell division by activating transcription of the major cluster of essential cell division genes of Escherichia coli. EMBO J. 10, 33633372.
  • [47]
    García-Lara, J., Shang, L.H., Rothfield, L.I. (1996) An extracellular factor regulates expression of sdiA, a transcriptional activator of cell division genes in Escherichia coli. J. Bacteriol. 178, 27422748.
  • [48]
    Joseleau-Petit, D., Vinella, D., D'Ari, D. (1999) Metabolic alarms and cell division in Escherichia coli. J. Bacteriol. 181, 914.
  • [49]
    Powell, B.S., Court, D.L. (1998) Control of ftsZ expression, cell division, and glutamine metabolism in Luria-Bertani medium by the alarmone ppGpp in Escherichia coli. J. Bacteriol. 180, 10531062.
  • [50]
    Navarro, F., Robin, A., D'Ari, R., Joseleau-Petit, D. (1998) Analysis of the effect of ppGpp on the ftsQAZ operon in Escherichia coli. Mol. Microbiol. 29, 815823.
  • [51]
    Wachi, M., Matsuhashi, M. (1989) Negative control of cell division by mreB, a gene that functions in determining the rod shape of Escherichia coli cells. J. Bacteriol. 171, 31233127.
  • [52]
    Gervais, F.G., Phoenix, P., Drapeau, G.R. (1992) The rcsB gene, a positive regulator of colanic acid biosynthesis in Escherichia coli, is also an activator of ftsZ expression. J. Bacteriol. 174, 39643971.
  • [53]
    Carballes, F., Bertrand, C., Bouché, J.P., Cam, K. (1999) Regulation of Escherichia coli cell division genes ftsA and ftsZ by the two component system rcsC-rcsB. Mol. Microbiol. 34, 442450.