Evidence for two promoters for the cytochrome c₂ gene (cycA) of Rhodobacter sphaeroides

Rhodobacter sphaeroides cytochrome c₂ (cyt c₂) is a periplasmic heme protein, encoded by cycA, that is required for photosynthetic growth and for one branch of the aerobic electron transport chain. cycA mRNA and cyt c₂ are more abundant photosynthetically than aerobically. We report here that there are four cycA transcripts by high-resolution Northern (RNA) blot analysis, and we have mapped 10 5' ends by primer extension. Complementation of a cycA null mutant shows that there are at least two cycA promoters: one within 89 bp upstream of the translation initiation codon for a transcript beginning at -28, and at least one within 484 bp upstream for the remaining nine 5' ends. The 5' ends at -28 and -137 are more abundant in aerobically grown cells, while those at -38, -155, -250, and -300 are more abundant photosynthetically. DNA sequences with homology to the Escherichia coli σ⁷⁰ consensus promoter sequence precede the 5' ends at -28 and -274, and there is weak homology upstream of the -82 and -250 ends.