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Formation of photosynthesis complexes in Rhodobacter sphaeroides is regulated in a redox- and light-dependent manner by the AppA/PpsR and PrrB/PrrA systems. While on the one hand, blue light is sensed by the flavin adenine dinucleotide-binding BLUF domain of AppA, on the other, light is absorbed by bacteriochlorophyll signals through PrrB/PrrA. We show that much smaller quantities initiate the AppA-mediated response to blue light than the bacteriochlorophyll-mediated response.

We provide experimental evidence for RNase III-dependent processing in helix 9 of the 23S rRNA as a general feature of many species in the alpha subclass of Proteobacteria (alpha-Proteobacteria). We investigated 12 Rhodobacter, Rhizobium, Sinorhizobium, Rhodopseudomonas, and Bartonella strains. The processed region is characterized by the presence of intervening sequences (IVSs). The 23S rDNA sequences between positions 109 and 205 (Escherichia coli numbering) were determined, and potential secondary structures are proposed. Comparison of the IVSs indicates very different evolutionary rates in some phylogenetic branches, lateral genetic transfer, and evolution by insertion and/or deletion. We show that the IVS processing in Rhodobacter capsulatus in vivo is RNase III-dependent and that RNase III cleaves additional sites in vitro. While all IVS-containing transcripts tested are processed in vitro by RNase III from R. capsulatus, E. coli RNase III recognizes only some of them as substrates and in these substrates frequently cleaves at different scissile bonds. These results demonstrate the different substrate specificities of the two enzymes. Although RNase III plays an important role in the rRNA, mRNA, and bacteriophage RNA maturation, its substrate specificity is still not well understood. Comparison of the IVSs of helix 9 does not hint at sequence motives involved in recognition but reveals that the "antideterminant" model, which represents the most recent attempt to explain the E. coli RNase III specificity in vitro, cannot be applied to substrates derived from alpha-Proteobacteria.

The bluF gene of Rhodobacter capsulatus is the first gene of the bluFEDCB operon which is involved in late steps of the cobalamin synthesis. To determine the function of the bluF gene product, a bluF::omega-Km mutant strain was constructed and characterized. This vitamin B12 auxotrophic mutant strain shows a 3.5-times higher vitamin B12 requirement under phototrophic growth conditions than under chemotrophic growth conditions. Surprisingly, the bluF promoter activity does not respond to alterations to the oxygen tension or vitamin B12 concentration.

A 6.4-kb region of a 6.8-kb BamHI fragment carrying Rhodobacter capsulatus genes involved in late steps of cobalamin synthesis has been sequenced. The nucleotide sequence and genetic analysis revealed that this fragment contains eight genes arranged in at least three operons. Five of these eight genes show homology to genes involved in the cobalamin synthesis of Pseudomonas denitrificans and Salmonella typhimurium. The arrangement of these homologous genes differs considerably in the three genera. Upstream of five overlapping genes (named bluFEDCB), a promoter activity could be detected by using lacZ fusions. This promoter shows no regulation by oxygen, vitamin B12 (cobalamin), or cobinamide. Disruption of the bluE gene by a Tn5 insertion (strain AH2) results in reduced expression of the puf and puc operons, which encode pigment-binding proteins of the photosynthetic apparatus. The mutant strain AH2 can be corrected to a wild-type-like phenotype by addition of vitamin B12 or cobinamide dicyanide. Disruption of the bluB gene by an interposon (strain BB1) also disturbs the formation of the photosynthetic apparatus. The mutation of strain BB1 can be corrected by vitamin B12 but not by cobinamide. We propose that a lack of cobalamin results in deregulation and a decreased formation of the photosynthetic apparatus.