Member of the two-component regulatory system BtsS/BtsR, which is part of a nutrient-sensing regulatory network composed of BtsS/BtsR, the low-affinity pyruvate signaling system YpdA/YpdB and their respective target proteins, BtsT and YhjX. Responds to depletion of nutrients, specifically serine, and the concomitant presence of extracellular pyruvate. BtsS is a high-affinity receptor for extracellular pyruvate that activates BtsR by phosphorylation. Activation of the BtsS/BtsR signaling cascade also suppresses YpdA/YpdB-mediated yhjX induction. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Interacts with BtsT and YhjX. Autophosphorylated. Deletion of btsSR has no obvious phenotypic effect under the conditions tested.