Promoter-specific involvement of the FixJ receiver domain in transcriptional activation

The “two-component” FixLJ system activates nitrogen fixation genes via nifA and fixK in Sinorhizobium meliloti. Like other response regulators, the FixJ protein can be decomposed into an N-terminal phosphorylatable “receiver” domain FixJN and a C-terminal transcriptional activator domain FixJC. The FixJN receiver domain was known to regulate activity of FixJC negatively at the nifA promoter. Here we show a different situation at the fixK promoter where FixJN also contributes positively to transcriptional activation. This promoter-specific effect was mapped by alanine-scanning mutagenesis to the β2 strand of the receiver domain. This interaction with FixJN is required for the recruitment of RNA polymerase at the fixK promoter by phosphorylated FixJ. Altogether the FixJ receiver domain appears to carry at least four functions, some of which can be separated by mutation: (1) autophosphorylation; (2) inhibition of FixJC; (3) dimerization; (4) transcriptional activation at pfixK. This example illustrates the formidable functional plasticity of receiver domains.