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). ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Expression of the rrpa-rrpB (nrsR-nrsS) operon is induced 3-fold by Ni(2+) and less by Co(2+). Autoregulates its own expression. Little visible phenotype, except up-regulation of nblA, a phycobilisome degradation protein (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).