Characterization of the ssnA gene, which is involved in the decline of cell viability at the beginning of stationary phase in Escherichia coli.

When grown in rich medium, Escherichia coli exhibits a drastic reduction of the number of viable cells at the beginning of stationary phase. The decline of cell viability was retarded by disruption of the ssnA gene, which was identified as a gene subject to RpoS-dependent negative regulation. Moreover, ssnA expression was induced at the time of decline of cell viability at early stationary phase. The viability decline was augmented in the rpoS background, and this augmentation was suppressed by ssnA mutation. Cloning of the ssnA gene in a multicopy plasmid, pBR322, caused small colony formation and slow growth in liquid medium. Cells harboring the ssnA clone showed aberrant morphology that included enlarged and filamentous shapes. The gene product was identified as a 44-kDa soluble protein, but its function could not be deduced by homology searching. From these results, we conclude that ssnA is expressed in response to a phase-specific signal(s) and that its expression level is controlled by RpoS, by a mechanism which may contribute to determination of cell number in the stationary phase.