Mutational analyses of two unique genes contained in the puf operon of the purple bacterium, Rubrivivax gelatinosus

Abstract

Apoproteins for the photosynthetic reaction center (RC) complex and the core light-harvesting (LH1) complex of purple photosynthetic bacteria form an operon called "puf". Although the structure of the operon (pufBALMC) is well conserved, that of a a -purple bacterium, Rubrivivax gelatinosus, additionally contains two characteristic ORFs, which locate upstream of pufB (orf48) and between pufB and pufA (orf74). The orf48 shows no sequence identities to known genes. The deduced amino acid sequence of the orf74 product shows 24% identity to the pufB product of this bacterium, although it lacks a putative bacteriochlorophyll-binding His residue and has an extended carboxyl-terminal tail. To elucidate the functions of these ORFs, two mutants lacking these genes were constructed (D orf48 and D orf74, respectively). The D orf74 cells showed a slightly slower growth rate than the parent cells. The BChl content per an active RC was also lowered although no reduction in the accumulation of mRNA for LH1 proteins was observed in the D orf74 cells. Spectrophotometric and biochemical analyses for the photosynthetic apparatus of the D orf74 cells are now in progress. The D orf48 cells showed approximately 4 times slower growth rate than the wild type, although the growth under the aerobic dark conditions was not affected. In this mutant, reduced production of the light-harvesting proteins (LH1) was observed. This was consistent with the observation that the accumulation of the short mRNA for pufB and pufA genes was extremely lowered. The orf48 may function in controlling the transcription of puf operon in Rvi. gelatinosus.