Member of the two-component regulatory system BvgS/BvgA. Phosphorylates BvgA via a four-step phosphorelay in response to environmental signals. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Activation requires a sequential transfer of a phosphate group from a His in the primary transmitter domain, to an Asp in the receiver domain and to a His in the secondary transmitter domain. Was originally thought to be two separate ORFs named bvgB and bvgC.