Two-component systems allow environmental conditions to be detected and transduced across the bacterial membrane, modulating gene expression.
Formed of two components (as implied by the name), the histidine kinase (HK) is extracellular, identifying the environmental change. The response regulator (RR) is intracellular, detecting the change in the HK and catalysing a reaction within the cell.
Two-component systems are involved in the acid response in E. coli, upregulating the expression of specific acid-response genes based on the extracellular H+ concentration, through the activation of σ38.