The RegA regulon exhibits variability in response to altered growth conditions and differs markedly between Rhodobacter species

The RegB/RegA two-component system from Rhodobacter capsulatus regulates global changes in gene expression in response to alterations in oxygen levels. Studies have shown that RegB/RegA controls many energy-generating and energy-utilizing systems such as photosynthesis, nitrogen fixation, carbon fixation, hydrogen utilization, respiration, electron transport and denitrification. In this report, we utilized RNA-seq and ChIP-seq to analyse the breadth of genes indirectly and directly regulated by RegA. A comparison of mRNA transcript levels in wild type cells relative to a RegA deletion strain shows that there are 257 differentially expressed genes under photosynthetic defined minimal growth medium conditions and 591 differentially expressed genes when grown photosynthetically in a complex rich medium. ChIP-seq analysis also identified 61 unique RegA binding sites with a well-conserved recognition sequence, 33 of which exhibit changes in neighbouring gene expression. These transcriptome results define new members of the RegA regulon including genes involved in iron transport and motility. These results also reveal that the set of genes that are regulated by RegA are growth medium specific. Similar analyses under dark aerobic conditions where RegA is thought not to be phosphorylated by RegB reveal 40 genes that are differentially expressed in minimal medium and 20 in rich medium. Finally, a comparison of the R. capsulatus RegA regulon with the orthologous PrrA regulon in Rhodobacter sphaeroides shows that the number of photosystem genes regulated by RegA and PrrA are similar but that the identity of genes regulated by RegA and PrrA beyond those involved in photosynthesis are quite distinct.