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The purple nonsulfur photosynthetic bacterium Rhodobacter capsulatus has been extensively studied for its metabolic versatility as well as for production of a gene transfer agent called RcGTA. Production of RcGTA is highest in the stationary phase of growth and requires the response regulator protein CtrA. The CtrA protein in Caulobacter crescentus has been thoroughly studied for its role as an essential, master regulator of the cell cycle. Although the CtrA protein in R. capsulatus shares a high degree of sequence similarity with the C. crescentus protein, it is nonessential and clearly plays a different role in this bacterium. We have used transcriptomic and proteomic analyses of wild-type and ctrA mutant cultures to identify the genes dysregulated by the loss of CtrA in R. capsulatus. We have also characterized gene expression differences between the logarithmic and stationary phases of growth. Loss of CtrA has pleiotropic effects, with dysregulation of expression of ~6% of genes in the R. capsulatus genome. This includes all flagellar motility genes and a number of other putative regulatory proteins but does not appear to include any genes involved in the cell cycle. Quantitative proteomic data supported 88% of the CtrA transcriptome results. Phylogenetic analysis of CtrA sequences supports the hypothesis of an ancestral ctrA gene within the alphaproteobacteria, with subsequent diversification of function in the major alphaproteobacterial lineages. doi: 10.1128/JB.00160-10

Purple aerobic anoxygenic phototrophs (AAPs) are the only organisms known to capture light energy to enhance growth only in the presence of oxygen but do not produce oxygen. The highly adaptive AAPs compose more than 10% of the microbial community in some euphotic upper ocean waters and are potentially major contributors to the fixation of the greenhouse gas C[O.sub.2]. We present the complete genomic sequence and feature analysis of the AAP Roseobacter denitrificans, which reveal clues to its physiology. The genome lacks genes that code for known photosynthetic carbon fixation pathways, and most notably missing are genes for the Calvin cycle enzymes ribulose bisphosphate carboxylase (RuBisCO) and phosphoribulokinase. Phyiogenetic evidence implies that this absence could be due to a gene loss from a RuBisCO-containing [alpha]-proteobacterial ancestor. We describe the potential importance of mixotrophic rather than autotrophie C[O.sub.2] fixation pathways in these organisms and suggest that these pathways function to fix C[O.sub.2] for the formation of cellular components but do not permit autotrophic growth. While some genes that code for the redox-dependent regulation of photosynthetic machinery are present, many light sensors and transcriptional regulatory motifs found in purple photosynthetic bacteria are absent.

The core of the photosynthetic apparatus of purple photosynthetic bacteria such as Rhodobacter capsulatus consists of a reaction center (RC) intimately associated with light-harvesting complex I (LH1) and the PufX polypeptide. The abundance of the RC and LH1 components was previously shown to depend on the product of the puhB gene (formerly known as orf214). We report here that disruption of puhB diminishes RC assembly, with an indirect effect on LH1 assembly, and reduces the amount of PufX. Under semiaerobic growth conditions, the core complex was present at a reduced level in puhB mutants. After transfer of semi-aerobically grown cultures to photosynthetic (anaerobic illuminated) conditions, the RC/LH1 complex became only slightly more abundant, and the amount of PufX increased as cells began photosynthetic growth. We discovered that the photosynthetic growth of puhB disruption strains of R. capsulatus starts after a long lag period, which is due to physiological adaptation rather than secondary mutations. Using a hybrid protein expression system, we determined that the three predicted transmembrane segments of PuhB are capable of spanning a cell membrane and that the second transmembrane segment could mediate self-association of PuhB. We discuss the possible function of PuhB as a dimeric RC assembly factor.

Research has been conducted on Rhodobacter capsulatus genomic sequence. The study of acyl-homoserine lactone signaling in R. capsulatus has been carried out, and the results demonstrate that cells concentrate this hydrophobic acyl-homoserine lactone and that this hydrophobic acyl-homoserine lactone activates gene transfer agent production.

In Rhodobacter capsulatus, genes encoding a putative histidine kinase (CckA) and response regulator (CtrA) are class I flagellar genes that together constituting signal trasduction system. Research shows that R. capsulatus cckA mutants exhibit reduced motility, while ctrA mutants are totally nonmotile, suggesting a variable amount of flagellar gene transcripts.

A new aerobic bacterial strain (JF-1) that contains photosynthetic pigment-protein complexes and which was isolated from black smoker plume waters of the Juan de Fuca Ridge was taxonomically and phylogenetically characterized. Strain JF-1 is a gram-negative, yellow-pigmented, motile bacterium that is salt-, pH- and thermotolerant. The taxonomic name of Citromicrobium bathyomarinum is suggested from strain JF-1.

The PufX protein was studied in an experiment wherein the heterologous expression of a pufX gene from Rhodobacter capsulatus was made in a strain of Rhodobacter sphaeroides that does not have the native pufX gene. Results show that light energy can be efficiently transduced by the R. sphaeroides strain featuring the PufX protein. It was concluded that the protein can play a role in the organization of a bacterial photosystem for the efficient transduction of energy.

Rhodobacter capsulatus is incapable of low- light phototrophic growth when the pufX gene has been deleted but suppressor mutants to the second codon of the pufX gene were able to use light energy, though less efficiently than the wild type. This partial restoration of the light energy usage is most likely due to the inefficient transfer of quinone between the B870 alpha polypeptide complex and the b-c1 complexes, a process potentially linked to the PufX protein which most likely reduces the number of B870 antenna Bchl molecules.

Bacteriophage-like gene transfer agents (GTAs) have been discovered in both of the prokaryotic branches of the three-domain phylogenetic tree of life. The production of a GTA (RcGTA) by the phototrophic alphaproteobacterium Rhodobacter capsulatus is regulated by quorum sensing and a phosphorelay homologous to systems in other species that control essential functions such as the initiation of chromosome replication and cell division. In wild-type strains, RcGTA is produced in <3% of cells in laboratory cultures. Mutants of R. capsulatus that exhibit greatly elevated production of RcGTA were created decades ago by chemical mutagenesis, but the nature and molecular consequences of the mutation were unknown. We show that the number of cells in a population that go on to express RcGTA genes is controlled by a stochastic process, in contrast to a genetic process. We used transposon mutagenesis along with a fluorescent protein reporter system and genome sequence data to identify a gene, rcc00280, that encodes an RTX family calcium-binding protein homologue. The Rc280 protein acts as an extracellular repressor of RcGTA gene expression by decreasing the percentage of cells that induce the production of RcGTA. [ABSTRACT FROM AUTHOR]