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CspA, the most characterized member of the  csp  gene family of  Escherichia coli , is highly expressed not only in response to cold stress, but also during the early phase of growth at 37°C. Here, we investigate at molecular level the antagonistic role played by the nucleoid proteins FIS and H-NS in the regulation of  cspA  expression under non-stress conditions. By means of both probing experiments and immunological detection, we demonstrate  in vitro  the existence of binding sites for these proteins on the  cspA  regulatory region, in which FIS and H-NS bind simultaneously to form composite DNA-protein complexes. While the  in vitro  promoter activity of  cspA  is stimulated by FIS and repressed by H-NS, a compensatory effect is observed when both proteins are added in the transcription assay. Consistently with these findings, inactivation of  fis  and  hns  genes reversely affect the  in vivo  amount of  cspA  mRNA. In addition, by means of strains expressing a high level of the alarmone guanosine tetraphosphate ((p)ppGpp) and  in vitro  transcription assays, we show that the  cspA  promoter is sensitive to (p)ppGpp inhibition. The (p)ppGpp-mediated expression of  fis  and  hns  genes is also analyzed, thus clarifying some aspects of the regulatory loop governing  cspA  transcription.

frontiers in molecular biosciences

An Interplay among FIS, H-NS, and Guanosine Tetraphosphate Modulates Transcription of the Escherichia coli cspA Gene under Physiological Growth Conditions