Synthesis and stability of reaction center polypeptides and implications for reaction center assembly in Rhodobacter sphaeroides.

The synthesis and stability of the H and M polypeptide subunits of the photosynthetic reaction center of Rhodobacter sphaeroides were examined. In wild-type cells, low levels of both M and H were found to be synthesized under aerobic growth conditions, with a 20-fold induction in their synthesis under anaerobic-dark growth conditions. Neither M nor H was stable in aerobic cells, but in anaerobic-dark grown cells stability increased dramatically with the t1/2 going from 4-6 min to values estimated in excess of 4 h. In the H-deficient mutant PUHA1, the synthesis and membrane partitioning of the M subunit were unaffected by the absence of the H subunit but there was a significant decrease in M polypeptide stability in the membranes of PUHA1 relative to wild-type cells. The stability of all or part of the M population in PUHA1 could be restored by providing an H-encoding plasmid in trans. It was also determined that the presence of bacteriochlorophyll in fully aerobic cells had a direct, positive effect on M and H stability, utilizing the oxygen-regulatory mutant strain T1a. Implications for the role of the H subunit and putative assembly factors in reaction center assembly are discussed.