The RC from purple bacteria is
comprised of L- and M-polypeptide subunits and A- and B-branches of
cofactors, respectively, with a pseudo-C2symmetry axis. An additional
H-subunit resides on the cytoplasmic side of the complex. The cofactor
arrangement in the RC of purple bacteria is shown inFig. 1with identical
cofactors along each branch of the RC, with the exception that some
species utilize different quinones. However, remarkably, charge
separation is unidirectional along the A-branch. The RC from Cfl.
aurantiacusshares similarities with the purple bacterial RC, however it
differs most significantly in containing three BChl and three BPh, and in
being comprised of only the L- and M- polypeptides . The cofactor arrangement
inCfl. aurantiacusRCs is generally thought to be as inFig. with the third
BPh presumed to replace BB. While there is no definitive proof
where the third BPh resides, circumstantial evidence that it occupies
the BBsite comes from sequence alignments of the (putative) L- and
M- polypeptides in comparison to the L- and M- polypeptides of purple
bacterial RCs. These alignments indicate that inCfl.aurantiacusand
Rfl. castenholzii: the His residue that serves as an axial ligand to BB
(residue M180 in B. viridis and Rb. capsulatus, and M182 in Rb.
sphaeroides) is leucine and isoleucine, respectively, and the His at
L153, which serves as the ligand to BA, is preserved . Furthermore,
The L- and M-subunits ofRfl. castenholziishow relatively high homology