FIGURE 9-6 Two-component regulatory systems. (a) At low cytoplasmic concentrations of glutamine, glutamine dissociates from NtrB, resulting in a conformational change that activates a protein kinase transmitter domain that transfers an ATP γ-phosphate to a conserved histidine (H) in the transmitter domain. This phosphate is then transferred to an aspartic acid (D) in the regulatory domain of the response regulator NtrC. This converts NtrC into its activated form, which binds the enhancer sites upstream of the glnA promoter (see Figure 9-5). (b) General organization of two-component histidyl-aspartyl phospho-relay regulatory systems in bacteria and plants. See A. H. West and A. M. Stock, 2001, Trends Biochem. Sci. 26:369.