Roles for the Rhodobacter sphaeroides CcmA and CcmG proteins

Identification of novel genes putatively involved in the photosystem synthesis of Bradyrhizobium sp. ORS 278

In aerobic anoxygenic phototrophs, oxygen is required for both the formation of the photosynthetic apparatus and an efficient cyclic electron transfer. Mutants of Bradyrhizobium sp. ORS278 affected in photosystem synthesis were selected by a bacteriochlorophyll fluorescence-based screening. Out of the 9,600 mutants of a random Tn5 insertion library, 50 clones, corresponding to insertions in 28 different genes, present a difference in fluorescence intensity compared to the WT. Besides enzymes and regulators known to be involved in photosystem synthesis, 14 novel components of the photosynthesis control are identified. Among them, two genes, hsIU and hsIV, encode components of a protein degradation complex, probably linked to the renewal of photosystem, an important issue in Bradyrhizobia which have to deal with harmful reactive oxygen species. The presence of homologs in non-photosynthetic bacteria for most of the regulatory genes identified during study suggests that they could be global regulators, as the RegA-RegB system.

Identification of proteins involved in formaldehyde metabolism by Rhodobacter sphaeroides

Formaldehyde is an intermediate formed during the metabolism of methanol or other methylated compounds. Many Gram-negative bacteria generate formaldehyde from methanol via a periplasmic pyrroloquinoline quinone (PQQ)-dependent dehydrogenase in which the alpha subunit of an alpha(2)beta(2) tetramer has catalytic activity. The genome of the facultative formaldehyde-oxidizing bacterium Rhodobacter sphaeroides encodes XoxF, a homologue of the catalytic subunit of a proposed PQQ-containing dehydrogenase of Paracoccus denitrificans. R. sphaeroides xoxF is part of a gene cluster that encodes periplasmic c-type cytochromes, including CycI, isocytochrome c(2) and CycB (a cyt c(553i) homologue), as well as adhI, a glutathione-dependent formaldehyde dehydrogenase (GSH-FDH), and gfa, a homologue of a glutathione-formaldehyde activating enzyme (Gfa). To test the roles of XoxF, CycB and Gfa in formaldehyde metabolism by R. sphaeroides, we monitored photosynthetic growth with methanol as a source of formaldehyde and whole-cell methanol-dependent oxygen uptake. Our data show that R. sphaeroides cells lacking XoxF or CycB do not exhibit methanol-dependent oxygen uptake and lack the capacity to utilize methanol as a sole photosynthetic carbon source. These results suggest that both proteins are required for formaldehyde metabolism. R. sphaeroides Gfa is not essential to activate formaldehyde, as cells lacking gfa are capable of both methanol-dependent oxygen uptake and growth with methanol as a photosynthetic carbon source.

Features of Rhodobacter sphaeroides CcmFH

In this study, the in vivo function and properties of two cytochrome c maturation proteins, CcmF and CcmH from Rhodobacter sphaeroides, were analyzed. Strains lacking CcmH or both CcmF and CcmH are unable to grow under anaerobic conditions where c-type cytochromes are required, demonstrating their critical role in the assembly of these electron carriers. Consistent with this observation, strains lacking both CcmF and CcmH are deficient in c-type cytochromes when assayed under permissive growth conditions. In contrast, under permissive growth conditions, strains lacking only CcmH contain several soluble and membrane-bound c-type cytochromes, albeit at reduced levels, suggesting that this bacterium has a CcmH-independent route for their maturation. In addition, the function of CcmH that is needed to support anaerobic growth can be replaced by adding cysteine or cystine to growth media. The ability of exogenous thiol compounds to replace CcmH provides the first physiological evidence for a role of this protein in thiol chemistry during c-type cytochrome maturation. The properties of R. sphaeroides cells containing translational fusions between CcmF and CcmH and either Escherichia coli alkaline phosphatase or beta-galactosidase suggest that they are each integral cytoplasmic membrane proteins with their presumed catalytic domains facing the periplasm. Analysis of CcmH shows that it is synthesized as a higher-molecular-weight precursor protein with an N-terminal signal sequence.