MUTATION OF THE SER2 CODON OF THE LIGHT-HARVESTING B870 ALPHA-POLYPEPTIDE OF RHODOBACTER-CAPSULATUS PARTIALLY SUPPRESSES THE PUFX PHENOTYPE

The exact function of the pufX gene product of Rhodobacter capsulatus is uncertain, but deletion of the pufX gene renders cells incapable of phototrophic growth on a minimal medium, and photosynthetic electron transfer is impaired in vitro. However, suppressor mutants that are ab le to grow phototropically are readily isolated. Two such suppressor m utants were characterized as to their phototrophic growth properties, their fluorescence at different incident light intensities, the integr ity of their chromatophores, and their abilities to generate a transme mbrane potential. We found that the photosynthetic apparatus in the su ppressor mutants was less stable than that of the pseudo-wild-type and primary mutant strains and that the suppressor mutants used light ene rgy less efficiently than the pseudo-wild-type strain. Therefore, the suppressor strains are more precisely designated partial suppressor mu tants, The locations and sequences of the suppressor mutations were de termined, and both were found to change the second codon of the pufA g ene, Pt is hypothesized that the serine residue specified by this codo n is important in interactions between the B870 alpha protein and othe r membrane-bound polypeptides and that suppressor mutations at this po sition partially compensate for loss of the PufX protein, A model is p roposed for the function of the PufX protein.