Member of the two-component regulatory system SrrA/SrrB, which is involved in the global regulation of staphylococcal virulence factors in response to environmental oxygen levels as well as biofilm formation. Also plays an essential role in host-derived nitric oxide resistance by regulating hmp/flavohemoglobin, an enzyme that detoxifies nitric oxide by converting it to nitrate. Functions as a sensor protein kinase which is autophosphorylated at a histidine residue and transfers its phosphate group to SrrA. In turn, SrrA binds to the upstream promoter regions of the target genes to positively and negatively regulate their expression. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Activated by SrrA. Expression is maximal during the postexponential phase of growth, particularly under microaerobic conditions.