Mutations in the Bacillus subtilis glnRA operon that cause nitrogen source-dependent defects in regulation of TnrA activity

The Bacillus subtilis nitrogen transcriptional factor TnrA is inactive in cells grown with excess nitrogen, e.g., glutamine or glutamate plus ammonium, because feedback-inhibited glutamine synthetase (product of glnA) binds to TnrA and blocks its DNA-binding activity, Two conditional mutations that allow TnrA-dependent gene expression in cells grown with glutamate plus ammonium, but not in glutamine-grown cells, were characterized. One mutant contained a mutation in the glnA ribosome binding site, while the other mutant synthesized it truncated GlnR protein that constitutively repressed glnRA expression. The levels of glutamine synthetase were reduced in both mutants. As a result, when these mutants are grown with excess nitrogen in the absence of glutamine, there is insufficient production of the feedback inhibitors necessary to convert glutamine synthetase into its feedback-inhibited form and TnrA-activated genes are expressed at high levels.