| Abstrakt: |
Rhodobacter sphaeroides rpoE encodes a 19.2 kDa protein, σE, related to members of the extra-cytoplasmic function subfamily of eubacterial RNA polymerase σ factors. We demonstrate that σE directs transcription from rpoE P1, the promoter for the rpoEchrR operon, and from cycA P3, a promoter for the cytochrome c2 structural gene. Comparison of these σE-dependent promoters reveals significant sequence conservation in their −35 and −10 regions; however, rpoE P1 is over 80-fold stronger than cycA P3. Both promoters contain identical −35 hexamers, −36TGATCC−31, that appear to constitute the preferred sequence, since any single base mutation in this region of cycA P3 reduces promoter function. The higher activity of rpoE P1 appears to reflect a better −10 region, −13TAAGA−9, as it contains four out of five of the nucleotides found to be important to σE-dependent transcription. We also propose that ChrR acts as an inhibitor of σE, since these two proteins can form a complex, and ΔchrR mutations increase σE-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 σE activity. |
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