OXYGEN-INSENSITIVE SYNTHESIS OF THE PHOTOSYNTHETIC MEMBRANES OF RHODOBACTER-SPHAEROIDES - A MUTANT HISTIDINE KINASE

Two new loci, prrB and prrC, involved in the positive regulation of ph otosynthesis gene expression in response to anaerobiosis, have been id entified in Rhodobacter sphaeroides. prrB encodes a sensor histidine k inase that is responsive to the removal of oxygen and functions throug h the response regulator PrrA. Inactivation of prrB results in a subst antial reduction of photosynthetic spectral complexes as well as in th e inability of cells to grow photosynthetically at low to medium light intensities. Together, prrB and prrA provide the major signal involve d in synthesis of the specialized intracytoplasmic membrane (ICM), har boring components essential to the light reactions of photosynthesis, Previously, J. K. Lee and S. Kaplan (J. Bacteriol. 174:1158-1171, 1992 ) identified a mutant which resulted in high-level expression of the p uc operon, encoding the apoproteins giving rise to the B800-850 spectr al complex, in the presence of oxygen as well as in the synthesis of t he ICM under conditions of high oxygenation, This mutation is shown to reside in prrB, resulting in a leucine-to-proline change at position 78 in mutant PrrB (PRRB78), Measurements of mRNA levels in cells conta ining the prrB78 mutation support the idea that prrB is a global regul ator of photosynthesis gene expression. Two additional mutants, PRRB1 and PRRB2, which make two truncated forms of the PrrB protein, possess substantially reduced amounts of spectral complexes. Although the pre cise role of prrC remains to be determined, evidence suggests that it too is involved in the regulatory cascade involving prrB and prrA. The genetic organization of the photosynthesis response regulatory (PRR) region is discussed.