Member of the two-component regulatory system PmrB/PmrA involved in regulation of virulence. Unphosphorylated PmrA represses extracellular enzyme genes. Phosphorylation of PmrA by PmrB relieves such repression, which leads to activation of extracellular enzyme genes. Phosphorylated PmrA seems to repress expression of the pmrCAB operon. Phosphorylated by PmrB.