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 Fi xJ 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. Thi s promoter-specific effect was mapped by alanine-scanning mutagenesis to th e beta2 strand of the receiver domain. This interaction with FixJN is requi red for the recruitment of RNA polymerase at the fixK promoter by phosphory lated FixJ. Altogether the FixJ receiver domain appears to carry at least f our functions, some of which can be separated by mutation: (1) autophosphor ylation; (2) inhibition of FixJC; (3) dimerization; (4) transcriptional act ivation at pfixK. This example illustrates the formidable functional plasti city of receiver domains. (C) 2001 Academic Press.