Member of two-component regulatory system RppA/RppB, involved in the establishment of the appropriate stoichiometry between the 2 photosystems. It senses changes in the plastoquinone (PQ) redox poise (PubMed:10894737). Another group shows this two-component pair, renamed NrsR/NrsS, controls the nickel-dependent expression of the nrsBACD operon; they suggest the photosystem-related activities seen earlier are due to the expression of NrsS (RppB) in the absence of its natural substrate NrsR (RppA) (PubMed:11849552). May accept phosphate from Hik2 in a possible Hik2/RppA two-component system (Probable). Interacts with histidine kinase Hik2; may accept phosphate from Hik2. Expression of the rrpa-rrpB (nrsR-nrsS) operon is induced 3-fold by Ni(2+) and less by Co(2+). Autoregulates its own expression. The N-terminal response regulatory domain inhibits DNA-binding by the rest of the protein, in its absence the protein binds specifically to the nrsRS-nrsBACD (slr0793-slr0796) intergenic region. Grows faster than wild-type photomixotrophically (in light with glucose), specific activity of photosystem II (PSII) is about 30% higher, most cells are single when 90% of wild-type are doublets. Increased transcription of most PSII genes under most conditions, transcription of PSI and phycobilisome-related genes are mostly decreased (PubMed:10894737). A double rppA-rppB (nrsR-nrsS) deletion is less tolerant to growth on Ni(2+), no longer expresses nrsB (slr0793, involved in Ni(2+) resistance) in response to Ni(2+). Loss of expression of this operon. There are no growth effects, no changes in pigment concentration, no changes in PSII or nblA transcript levels seen in the double mutant (PubMed:11849552).