Massive presence of the Escherichia coli ‘majorcold‐shock protein’ CspA under non‐stress conditions

The most characteristic event of cold‐shock activation in Escherichia coli is believed to be the de novo synthesis of CspA. We demonstrate, however, that the cellular concentration of this protein is ≥50 μM during early exponential growth at 37°C; therefore, its designation as a major cold‐shock protein is a misnomer. The cspA mRNA level decreases rapidly with increasing cell density, becoming virtually undetectable by mid‐to‐late exponential growth phase while the CspA level declines, although always remaining clearly detectable. A burst of cspA expression followed by a renewed decline ensues upon dilution of stationary phase cultures with fresh medium. The extent of cold‐shock induction of cspA varies as a function of the growth phase, being inversely proportional to the pre‐existing level of CspA which suggests feedback autorepression by this protein. Both transcriptional and post‐transcriptional controls regulate cspA expression under non‐stress conditions; transcription of cspA mRNA is under the antagonistic control of DNA‐binding proteins Fis and H‐NS both in vivo and in vitro , while its decreased half‐life with increasing cell density contributes to its rapid disappearance. The cspA mRNA instability is due to its 5′ untranslated leader and is counteracted in vivo by the cold‐shock DeaD box RNA helicase (CsdA).