A novel membrane‐associated c‐type cytochrome, cyt cy, can mediate the photosynthetic growth of Rhodobacter capsulatus and Rhodobacter sphaeroides.

Mutants of Rhodobacter capsulatus lacking the soluble electron carrier cytochrome c2 are able to grow photosynthetically (Ps+), whereas Rhodobacter sphaeroides is unable to do so. To understand this unusual electron transfer pathway the gene required for cyt c2‐independent growth of R.capsulatus was sought using chromosomal libraries derived from a cyt c2‐ mutant of this species to complement a Ps‐ cyt c2‐ mutant of R.sphaeroides to Ps+ growth. The complementing 1.2 kbp DNA fragment contained a gene, cycY, encoding a novel membrane‐associated c‐type cytochrome, cyt cy, based on predicted amino acid sequence, optical difference spectra and SDS‐PAGE analysis of chromatophore membranes. The predicted primary sequence of cyt cy is unusual in having two distinct domains, a hydrophobic amino‐terminal region and a carboxyl‐terminus with strong homology to cytochromes c. A cyt cy‐ mutant of R.capsulatus remains Ps+ as does the cyt c2‐ mutant. However, a mutant lacking both cyt c2 and cy is Ps‐, and can be complemented to Ps+ by either cyt c2 or cyt cy. These findings demonstrate that each of the cytochromes c2 and cy is essential for photosynthesis only in the absence of the other. Thus, two distinct electron transfer pathways, unrecognized until now, operate during photosynthesis in R.capsulatus under appropriate conditions, one via the soluble cyt c2 and the other via the membrane‐associated cyt cy.