A GLOBAL SIGNAL-TRANSDUCTION SYSTEM REGULATES AEROBIC AND ANAEROBIC CO2 FIXATION IN RHODOBACTER-SPHAEROIDES

Complementation of a mutant of Rhodobacter sphaeroides defective in ph otosynthetic CO2 reduction led to the identification of a gene which e ncodes a protein that is related to a class of sensor kinases involved in bacterial signal transduction. The nucleotide sequence and deduced amino acid sequence led to the finding that the gene which complement ed the mutant is the regB (prrB) gene, previously isolated from both R . sphaeroides and Rhodobacter capsulatus and shown to regulate the ana erobic expression of structural genes required for the synthesis of th e reaction center and light-harvesting systems of these organisms. The current investigation indicates that in addition to its role in the r egulation of photosystem biosynthesis, regB (prrB) of X. sphaeroides i s intimately involved in the positive regulation of the cbb(I) and cbb (II) Calvin cycle CO2 fixation operons. In addition to regulating the expression of structural genes encoding enzymes of the primary pathway for CO2 fixation in R. sphaeroides, regB was also found to be require d for the expression of a gene(s) important for the putative alternati ve CO2 fixation pathway(s) of this organism. A mutation in regB also b locked expression of structural genes of the cbb regulon in a strain o f R. sphaeroides capable of aerobic CO2-dependent growth in the dark. It is thus apparent that regB is part of a two-component system and en codes a sensor kinase involved in the global regulation of both anoxyg enic light-dependent- and oxygenic light-independent CO2 fixation as w ell as anoxygenic photosystem biosynthesis.