Induction of competence for natural genetic transformation in Streptococcus pneumoniae depends on a signalling pathway consisting of the competence-stimulating peptide (CSP) (Håvarstein et al., 1995) and a two-component regulatory system (TCS), ComD–ComE (Pestova et al., 1996). The interaction between the secreted CSP and its membrane-embedded histidine kinase (HK) receptor ComD is believed to bring on a conformational change in the ComD transmembrane domain that ultimately results in activation of ComD kinase activity. A comparison of highly homologous ComD receptors, which are specific for different but closely related pheromone types, show that they are most divergent in the first 60–100 N-terminal aa residues. This part of the ComD receptor, which contains two or three membrane-spanning segments, is likely to determine the specificity of the receptor–ligand interaction (Håvarstein et al., 1996). According to a current model, once activated, ComD phosphorylates its cognate response regulator (RR) ComE. ComE∼P then activates transcription of the early competence (com) genes (Claverys and Håvarstein, 2002; Claverys et al., 2006). These genes include the comCDE (Fig. 1, Top) and comAB operons. The latter encodes the machinery required for maturation and export of the comC-encoded pre-CSP (Hui et al., 1995). Thus, by increasing CSP production and export capacity, this transcriptional activation promotes the autocatalytic accumulation of CSP that presumably accounts for the rapid development and synchronization of competence in pneumococcal cultures. The early com genes also include comX, which encodes an alternative sigma factor (Lee and Morrison, 1999) that directs the transcription of a large number of so-called late com genes. The latter includes genes required for the uptake and processing of transforming DNA into recombinants (Claverys and Håvarstein, 2002; Claverys et al., 2006).
The presence of a direct repeat in the comCDE promoter region, which is bound by ComE in vitro (CEbs) (Ween et al., 1999), and the location of the start of CSP-induced comCDE transcripts (Halfmann et al., 2007) with respect to CEbs (Fig. 1, Middle) are consistent with the model of ComE-dependent transcriptional activation of comCDE in response to CSP. On the other hand, while it is common to assume a ‘basal level of expression’ of the competence pheromone and of the TCS that responds to it, no one has previously defined basal expression of either genes, nor established how it might be regulated.
Here, we provide evidence that non-autocatalytic comCDE transcription (i.e. expression outside the transcriptional burst co-ordinated by CSP; we will also refer to this as basal expression) relies on the same promoter element (−10 box) as CSP-induced expression. We identify two proteins important for the basal expression, ComD and ComE, and show that non-autocatalytic comCDE expression necessitates the presence of a phosphate-accepting form of ComE, but not CSP. These observations imply that self-activation of ComD is responsible for the non-autocatalytic expression of comCDE. We also document the occurrence of transcriptional readthrough across the transcription terminator of tRNAArg5 upstream of comCDE. This finding establishes that two (possibly three) promoters drive basal expression of the comCDE operon. Finally, we provide evidence that this readthrough is crucial to counteract the accidental production of cells devoid of ComE or ComD, which would otherwise be unable to reactivate the competence cascade or respond to CSP.