The Rhodobacter sphaeroides ECF sigma factor, sigma(E), and the target promoters cycA P3 and rpoE P1

Rhodobacter sphaeroides rpoE encodes a 19.2 kDa protein, sigma(E), related to members of the extra-cytoplasmic function subfamily of eubacterial RNA p olymerase sigma factors. We demonstrate that sigma(E) directs transcription from rpoE P1, the promoter for the rpoEchrX operon, and from cycA P3, a pr omoter for the cytochrome c(2) structural gene. Comparison of these sigma(E )-dependent promoters reveals significant sequence conservation in their -3 5 and -10 regions; however, rpoE P1 is over 80-fold stronger than cycA P3. Both promoters contain identical -35 hexamers, (-36)TGATCC(-31), that appea r to constitute the preferred sequence, since any single base mutation in t his region of cycA P3 reduces promoter function. The higher activity of rpo E P1 appears to reflect a better -10 region, (-13)TAAGA(-9), as it contains four out of five of the nucleotides found to be important to sigma(E)-depe ndent transcription. We also propose that ChrR acts as an inhibitor of sigm a(E), since these two proteins can form a complex, and Delta chrR mutations increase sigma(E)-dependent transcription. ChrR is believed to respond to a signal from tetrapyrrole biosynthesis because loss of function mutations in chrR lead to cohemin resistance. Based on our observations, we present a model in which cohemin resistance is conferred by increasing sigma(E) acti vity. (C) 1999 Academic Press.