Regulated expression of a highly conserved regulatory gene cluster is necessary for controlling photosynthesis gene expression in response to anaerobiosis in Rhodobacter capsulatus

We utilized primer extension analysis to demonstrate that the divergently t ranscribed regB and senC-regAhvrA transcripts contain stable 5' ends 43 nuc leotides apart within the regB-senC intergenic region. DNA sequence analysi s indicates that this region contains two divergent promoters with overlapp ing sigma(70) type -35 and -10 promoter recognition sequences. In vivo anal ysis of expression patterns of regB::lacZ and senC-regA-hvrA::lacZ reporter gene fusions demonstrates that the regB and senC-regA-hvrA transcripts are both negatively regulated by the phosphorylated form of the global respons e regulator RegA. DNase I protection assays with a constitutively active va riant of RegA indicate that RegA binds between regB and senC overlapping -1 0 and -35 promoter recognition sequences. Two mutations were also isolated in a regB deficient background that increased expression of the senC-regA-h vrA operon 10- and 5-fold, respectively. As a consequence of increased RegA expression, these mutants exhibited elevated aerobic and anaerobic photosy nthesis (puf) gene expression, even in the absence of the sensor kinase Reg B, These results indicate that autoregulation by RegA is a factor contribut ing to the maintenance of an optimal low level of RegA expression that allo ws responsiveness to activation by phosphorylation.