OXYGEN-REGULATED IN-VITRO TRANSCRIPTION OF RHIZOBIUM-MELILOTI NIFA AND FIXK GENES

Oxygen concentration regulates the expression of nitrogen fixation gen es in the symbiotic bacterium Rhizobium meliloti. We demonstrate that two proteins, FixL and FixJ, that belong to the two-component family o f regulatory proteins are necessary and sufficient for oxygen-regulate d in vitro transcription of the two key regulatory genes, nifA and fix K. We show directly that FixJ is a transcriptional activator, working in conjunction with the RNA polymerase sigma 70 holoenzyme. Addition o f FixL122, a soluble form of the sensor FixL protein, to the transcrip tion assay enhanced FixJ transcriptional activity in response to low o xygen concentration. This enhancement of FixJ activity was correlated with FixJ phosphorylation.