The photosynthetic deficiency due to puhC gene deletion in Rhodobacter capsulatus suggests a PuhC protein-dependent process of RC/LH1/PufX complex reorganization

Optimal photosynthetic reaction centre (RC) and core antenna (LH1) levels in the purple bacterium Rhodobacter capsulatus require the puhC gene. Deletion of puhC had little effect on RC and LH1 assembly individually, but significantly inhibited the photosynthetic growth of RC+LH1− strains, suggesting that maximal RC catalytic activity is PuhC-dependent. Consistent with post-assembly reorganization of the RC/LH1/PufX core complex by PuhC to include latecomer proteins, spatial separation of pufX from the RC/LH1 genes inhibited PufX accumulation and photosynthetic growth only in PuhC− strains. Photosynthetic activity improved to different degrees when PuhC homologues from three other species were expressed in PuhC−R. capsulatus, indicating that PuhC homologues function similarly but may interact inefficiently with a heterologous core complex. Anaerobic photosynthetic growth of PuhC− strains was affected by the duration of prior semiaerobic growth, and by two genes that modulate bacteriochlorophyll production: pufQ and puhE. These observations agree with a speculative model in which reorganization of the core complex is an important regenerative process, accelerated by PuhC.