Massive presence of the Escherichia coli 'major cold-shock protein' CspA under non-stress conditions

The most characteristic event of cold-shock activation in Escherichia coil is believed to be the de novo synthesis of CspA. We demonstrate, however, t hat the cellular concentration of this protein is greater than or equal to 50 mu M during early exponential growth at 37 degrees C; therefore, its des ignation as a major cold-shock protein is a misnomer, The cspA mRNA level d ecreases rapidly with increasing cell density, becoming virtually undetecta ble by mid-to-late exponential growth phase while the CspA level declines, although always remaining clearly detectable. A burst of cspA expression fo llowed by a renewed decline ensues upon dilution of stationary phase cultur es 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-exi sting level of CspA which suggests feedback autorepression by this protein, Both transcriptional and post-transcriptional controls regulate cspA expre ssion 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 contri butes 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 bo x RNA helicase (CsdA).