Evidence for a functional similarity between the two-component regulatory systems RegSR, ActSR, and RegBA (PrrBA) in alpha-proteobacteria

The symbiotic bacteria Bradyrhizobium japonicum and Sinorhizobium meliloti, and the purple photosynthetic bacteria Rhodobacter capsulatus, Rhodovulum sulfidophilum, Roseobacter denitrificans and Rhodobacter sphaeroides posses s homologous two-component regulatory systems, namely RegSR, ActSR, RegBA a nd PrrBA. The respective response regulators of these bacteria control expr ession of different regulons that are involved in N-2 fixation, CO2 fixatio n, photosynthesis or acid tolerance. We therefore asked whether the regulat ors are functionally exchangeable or whether they have disparate functions in the different species, despite the amino acid sequence similarity. In th is study, we showed that purified B. japonicum RegR bound in vitro to genui ne DNA targets for Rba. capsulatus RegA, and that RegA was phosphorylated i n vitro when RegS(C) (a soluble variant of the sensor kinase RegS) was adde d to an Escherichia coli extract containing overexpressed RegA. In vivo, Re gA and S. meliloti ActR activated transcription of the B. japonicum fixR-ni fA operon, normally a target for RegR. The genes for both regulators, regA and actR, were able to complement a B. japonicum regR mutant with respect t o the formation of a nitrogen-fixing symbiosis with soybean. Vice versa, Re gR activated in Rba. capsulatus the expression of the photosynthesis operon puc, normally a target for RegA. In conclusion, the results shaw that B. j aponicum RegR, Rba. capsulatus RegA, and S. meliloti ActR are functionally similar.