Exchange and complementation of genes coding for photosynthetic reaction center core subunits among purple bacteria

A mutant of the phototrophic species belonging to the β-proteobacteria, Rubrivivax gelatinosus, lacking the photosynthetic growth ability was constructed by the removal of genes coding for the L, M, and cytochrome subunits of the photosynthetic reaction center complex. The L, M, and cytochrome genes derived from five other species of proteobacteria, Acidiphilium rubrum, Allochromatium vinosum, Blastochloris viridis, Pheospirillum molischianum, and Roseateles depolymerans, and the L and M subunits from two other species, Rhodobacter sphaeroides and Rhodopseudomonas palustris, respectively, have been introduced into this mutant. Introduction of the genes from three of these seven species, Rte. depolymerans, Ach. vinosum, and Psp. molischianum, restored the photosynthetic growth ability of the mutant of Rvi. gelatinosus, although the growth rates were 1.5, 9.4, and 10.7 times slower, respectively, than that of the parent strain. Flash-induced kinetic measurements for the intact cells of these three mutants showed that the photo-oxidized cytochrome c bound to the introduced reaction center complex could be rereduced by electron donor proteins of Rvi. gelatinosus with a t1/2 of less than 10 ms. The reaction center core subunits of photosynthetic proteobacteria appear to be exchangeable if the sequence identities of the LM core subunits between donor and acceptor species are high enough, i.e., 70% or more.