The chapter discusses the role of protein phosphorylation in regulating adaptive responses in bacteria, focusing on the mechanisms and components involved. Adaptive responses in bacteria range from rapid changes in motility to long-term global reorganizations of gene expression and cell morphology. These responses are controlled by signals from within the cytoplasm and the environment, involving two main types of enzymatic components: histidine protein kinases (HPKs) and response regulators (RRs). Signal transduction occurs through the transfer of phosphoryl groups from ATP to HPKs, followed by transfer to RR aspartyl residues, and finally to water. The level of phosphorylation of an RR controls its activity, and the regulation of this process can control adaptive responses. The chapter also covers the chemistry of protein phosphorylation, including phosphorylation at imidazole nitrogen and aspartate residues, and the role of phosphatases in dephosphorylation. It discusses specific systems such as chemotaxis, nitrogen regulation, phosphate regulation, and osmoregulation, detailing the molecular mechanisms and genetic control involved. The text highlights the cross-talk between different signal transduction pathways and the integration of various sensory inputs to coordinate diverse output strategies. Finally, the chapter reviews the structure and function of HPKs and RRs, emphasizing their conserved domains and the importance of phosphorylation in their activity. It also explores the role of auxiliary regulatory proteins, such as CheZ, in enhancing the rate of dephosphorylation of phosphorylated RRs. The chapter concludes by discussing the physiological and genetic aspects of chemotaxis, including the interaction between Che proteins and the flagellar motor, and the role of ATP in controlling flagellar rotation.The chapter discusses the role of protein phosphorylation in regulating adaptive responses in bacteria, focusing on the mechanisms and components involved. Adaptive responses in bacteria range from rapid changes in motility to long-term global reorganizations of gene expression and cell morphology. These responses are controlled by signals from within the cytoplasm and the environment, involving two main types of enzymatic components: histidine protein kinases (HPKs) and response regulators (RRs). Signal transduction occurs through the transfer of phosphoryl groups from ATP to HPKs, followed by transfer to RR aspartyl residues, and finally to water. The level of phosphorylation of an RR controls its activity, and the regulation of this process can control adaptive responses. The chapter also covers the chemistry of protein phosphorylation, including phosphorylation at imidazole nitrogen and aspartate residues, and the role of phosphatases in dephosphorylation. It discusses specific systems such as chemotaxis, nitrogen regulation, phosphate regulation, and osmoregulation, detailing the molecular mechanisms and genetic control involved. The text highlights the cross-talk between different signal transduction pathways and the integration of various sensory inputs to coordinate diverse output strategies. Finally, the chapter reviews the structure and function of HPKs and RRs, emphasizing their conserved domains and the importance of phosphorylation in their activity. It also explores the role of auxiliary regulatory proteins, such as CheZ, in enhancing the rate of dephosphorylation of phosphorylated RRs. The chapter concludes by discussing the physiological and genetic aspects of chemotaxis, including the interaction between Che proteins and the flagellar motor, and the role of ATP in controlling flagellar rotation.