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. BtsR regulates expression of btsT by binding to its promoter region. Activation of the BtsS/BtsR signaling cascade also suppresses YpdA/YpdB-mediated yhjX induction. Phosphorylated by BtsS. Deletion of btsSR has no obvious phenotypic effect under the conditions tested. Extended N-terminus.