Effect of AmtB homologues on the post-translational regulation of nitrogenase activity in response to ammonium and energy signals in Rhodospirillum rubrum

The AmtB protein transports uncharged NH₃ into the cell, but it also interacts with the nitrogen regulatory protein P_ll, which in turn regulates a variety of proteins involved in nitrogen fixation and utilization. Three P_ll homologues, GlnB, GlnK and GlnJ, have been identified in the photosynthetic bacterium Rhodospirillum rubrum, and they have roles in at least four overlapping and distinct functions, one of which is the post-translational regulation of nitrogenase activity. In R. rubrum, nitrogenase activity is tightly regulated in response to NH₄⁺ addition or energy depletion (shift to darkness), and this regulation is catalysed by the post-translational regulatory system encoded by draTG. Two amtB homologues, amtB₁ and amtB₂, have been identified in R. rubrum, and they are linked with glnJ and glnK, respectively. Mutants lacking AmtB₁ are defective in their response to both NH₄⁺ addition and darkness, while mutants lacking AmtB₂ show little effect on the regulation of nitrogenase activity. These responses to darkness and NH₄ ⁺ appear to involve different signal transduction pathways, and the poor response to darkness does not seem to be an indirect result of perturbation of internal pools of nitrogen. It is also shown that AmtB₁ is necessary to sequester detectable amounts GlnJ to the cell membrane. These results suggest that some element of the AmtB₁-P_ll regulatory system senses energy deprivation and a consistent model for the integration of nitrogen, carbon and energy signals by P_ll is proposed. Other results demonstrate a degree of specificity in interaction of AmtB₁ with the different P_ll homologues in R. rubrum. Such interaction specificity might be important in explaining the way in which P_ll proteins regulate processes involved in nitrogen acquisition and utilization.