Expression of Escherichia coli cspA during early exponential growth at 37 °C.

CspA is a small (7.4 kDa) nucleic acid binding protein of Escherichia coli whose expression is stimulated after cold-stress but whose level is also extraordinarily high during the early phase of growth of non-stressed cells. In this study the relationship existing between cspA transcription/translation on the one hand and the acquisition of critical mass for cell division and chromosome replication, on the other, in stationary phase cells subjected to a nutritional up-shift at 37 °C has been analyzed. Measurements of optical density and viable counts, pulse-chase, real-time PCR and immunodetection experiments, as well as cytofluorimetric and DNA duplication analyses show that synthesis of new CspA molecules at 37 °C is not only restricted to the lag phase ensuing the nutritional up-shift, but continues also during the first stages of logarithmic growth, when cells have already started dividing; although the early synthesized molecules are diluted by the following cell divisions and new synthesis occurs at an extremely low level, cspA mRNA and CspA continue to be present. A possible explanation for the apparent paradox that cspA is activated not only following cold stress, but also under non-stress and other stress conditions which entail a down-regulation of bulk gene expression and protein synthesis is presented.