Isolation of regulatory mutants in photosynthesis gene expression in Rhodobacter sphaeroides 2.4.1 and partial complementation of a PrrB mutant by the HupT histidine-kinase.

The photosynthetic bacterium Rhodobacter sphaeroides responds to the transition from aerobiosis to anaerobic photosynthesis by increasing the expression of the photosynthesis genes. Mutants have been isolated based on their inability, following such a transition, to increase transcription of the puc operon encoding the apoproteins of the light-harvesting complex II. Mutant D5, a representative of one mutant class, described here, although remaining photosynthetically competent, produced only low levels of the photosynthetic spectral complexes. Complementation analysis revealed that either the gene for the photosynthesis response regulator prrA or the gene encoding its cognate sensor kinase, prrB, was capable of rescuing this mutant. However, partial complementation of this mutant was achieved by placing in trans additional copies of other defined genes from the cosmid library of R. sphaeroides. We describe this effect in detail, attributable to the hupT gene, which has been proposed to encode a histidine-kinase for the hydrogen uptake system in Rhodobacter capsulatus. The effect of HupT on the expression of the photosynthesis genes was mediated through PrrA and independent of a functioning hydrogen uptake system. Thus, we raise the possibility that HupT can participate in phosphorylation of the heterologous response regulator PrrA by so-called cross-talk and therefore partially compensate for the defect in the mutant described. The observation of cross-talk, together with the complementation analysis, allowed us to assign the original mutation to the prrB gene; this was confirmed by DNA sequencing. Analysis of cross-talk in the wild-type, prrB and prrA genetic backgrounds suggested that besides kinase activity, PrrB may possess phosphatase activity toward PrrA. We also report the cloning, organization and structure of some of the hup genes from R. sphaeroides and construction of a Hup- strain.