CsrA post-transcriptionally represses pgaABCD, responsible for synthesis of a biofilm polysaccharide adhesin of Escherichia coli

Authors

  • Xin Wang,

    1. Department of Microbiology and Immunology, Emory University School of Medicine, 3105 Rollins Research Center, 1510 Clifton Road N.E., Atlanta, GA 30322, USA.
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  • Ashok K. Dubey,

    1. Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA 16802, USA.
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  • Kazushi Suzuki,

    1. Department of Microbiology and Immunology, Emory University School of Medicine, 3105 Rollins Research Center, 1510 Clifton Road N.E., Atlanta, GA 30322, USA.
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  • Carol S. Baker,

    1. Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA 16802, USA.
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  • Paul Babitzke,

    1. Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA 16802, USA.
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  • Tony Romeo

    Corresponding author
    1. Department of Microbiology and Immunology, Emory University School of Medicine, 3105 Rollins Research Center, 1510 Clifton Road N.E., Atlanta, GA 30322, USA.
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E-mail romeo@microbio.emory.edu; Tel. (+1) 404 727 3734; Fax (+1) 404 727 3659.

Summary

The RNA-binding protein CsrA represses biofilm formation, while the non-coding RNAs CsrB and CsrC activate this process by sequestering CsrA. We now provide evidence that the pgaABCD transcript, required for the synthesis of the polysaccharide adhesin PGA (poly-β-1,6-N-acetyl-d-glucosamine) of Escherichia coli, is the key target of biofilm regulation by CsrA. csrA disruption causes an approximately threefold increase in PGA production and an approximately sevenfold increase in expression of a pgaA′–′lacZ translational fusion. A ΔcsrBΔcsrC mutant exhibits a modest decrease in pgaA′–′lacZ expression, while the response regulator UvrY, a transcriptional activator of csrB and csrC, stimulates this expression. Biofilm formation is not regulated by csrA, csrB or uvrY in a ΔpgaC mutant, which cannot synthesize PGA. Gel mobility shift and toeprint analyses demonstrate that CsrA binds cooperatively to pgaA mRNA and competes with 30S ribosome subunit for binding. CsrA destabilizes the pgaA transcript in vivo. RNA footprinting and boundary analyses identify six apparent CsrA binding sites in the pgaA mRNA leader, the most extensive arrangement of such sites in any mRNA examined to date. Substitution mutations in CsrA binding sites overlapping the Shine–Dalgarno sequence and initiation codon partially relieve repression by CsrA. These studies define the crucial mechanisms, though not the only means, by which the Csr system influences biofilm formation.

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