Photosynthetic deficiency of a pufX deletion mutant of Rhodobacter sphaeroides is suppressed by point mutations in the light-harvesting complex genes pufB or pufA.

The pufX gene of the facultative phototroph Rhodobacter sphaeroides encodes a membrane protein that is required for photoheterotrophic growth. Deletion of pufX impairs the photosynthetic generation of a transmembrane potential, suggesting a role for the PufX protein in light-driven cyclic electron transfer [Farchaus, J. W., et al. (1992) EMBO J. 11, 2779-2788]. Here we describe the isolation and characterization of 65 spontaneous suppressor mutants in which photosynthetic competence was restored by secondary mutations. Genetic analysis revealed the occurrence of single point mutations altering highly conserved residues within the light-harvesting complex, B875. One of three tryptophan codons was changed to stop or arginine codons in 89% of these suppressor mutants. Spectral characterization and Western blot analysis were used to examine the B875 assembly and the stable expression of the altered light-harvesting polypeptides. Three different groups of suppressor mutants were found: (1) No stable expression of altered B875 polypeptides was detected for the alpha 43W-->* and beta 44W-->* mutants. (2) There was expression of the mutated B875-beta chain, but no stable B875 assembly in the beta 47W-->R mutants. (3) Intact B875 complexes were found for the alpha 47S-->F or beta 20H-->R mutants. These results provide evidence that the differently altered B875 polypeptides do not substitute directly for the PufX protein but lead to structural rearrangements in the macromolecular membrane organization, thus restoring a sufficiently high capacity for light-driven cyclic electron transfer.