The FixJ transcriptional activator: from structure to genome

摘要

In the alfalfa symbiont Sinorhizobium meliloti, nitrogen fixation genes are controlled by the oxygen-regulated FixLJ 'two-component' regulatory system. Under microoxic conditions as provided by root nodules, the FixL histidine kinase phosphorylatesFixJ, turning it into a transcriptional activator of the nifA and fixK promoters. As a consequence a genetic cascade is switched on, allowing expression of the nitrogen fixation apparatus during symbiosis (for review see Fischer 1994). Like other response regulators, FixJ is a modular protein, which has been dissected into an N-terminal phosphorylatable 'receiver' domain FixJN and a C-terminal transcriptional activator domain FixJC. In its non-phosphorylated form, the FixJN receiver domain inhibits thelatent activity of FixJC at the nifA promoter (Kahn, Ditta 1991; Da Re et al. 1994). Phosphorylation simultaneously relieves this inhibition and triggers the dimerization of the protein via the FixJN receiver domain (Figure 1), resulting in a FixJ approxP dimer which is the active form of FixJ (Da Re et al. 1999).