Transcriptional induction of the conserved alternative sigma factor RpoS in Escherichia coli is dependent on BarA, a probable two-component regulator

The stationary phase expression of many conserved, adaptive bacterial prote ins is dependent on RpoS, a second vegetative sigma factor. The regulation of RpoS itself, however, is complex and not fully understood, particularly at the level of transcription. In this report, we show that the observed hy drogen peroxide sensitivity of a mutant defective in expression of barA, a bacterial virulence factor, can be explained by a reduction in catalase act ivity, an RpoS-controlled function. Levels of katE mRNA, encoding the major catalase of Escherichia coli, were much lower in the barA mutant, suggesti ng that BarA is required for the expression of this RpoS-regulated gene. Ex pression of another RpoS-regulated gene, osmY, was also found to be severel y reduced in the barA mutant. Employing Western analyses with anti-RpoS ant isera and Northern analyses using probes specific for rpoS, we found that B arA is required for the exponential phase induction of RpoS itself. Operon lacZ fusion expression studies and Northern analyses indicate that BarA its elf is maximally expressed in early exponential phase cultures immediately preceding the transcriptional induction of RpoS. Results of primer extensio n studies indicate that exponential phase expression from the rpoSp1 promot er is reduced by more than 85% in a barA mutant but could be efficiently co mplemented by a plasmid-borne copy of barA in trans. These results suggest that regulatory signals that are operant in exponentially growing cultures play an important role in effecting stationary phase gene expression.