Explain bacterial two-component signaling systems.

Two-component systems are the predominant signal transduction mechanism in bacteria, consisting of a sensor histidine kinase (HK) and a response regulator (RR). The HK, often membrane-bound, autophosphorylates a conserved histidine residue upon signal detection. This phosphate is transferred to a conserved aspartate on the RR, typically activating its DNA-binding domain to alter gene expression. Examples include EnvZ-OmpR (osmolarity response), PhoR-PhoB (phosphate limitation), and CheA-CheY (chemotaxis). These systems enable rapid environmental responses and are absent in mammals, making them potential antibiotic targets.