Inhibition of the FixL sensor kinase by the FixT protein in Sinorhizobium meliloti

Nitrogen fixation in symbiotic rhizobia is subject to multiple levels of ge ne regulation. In Sinorhizobium meliloti, the alfalfa symbiont, the FixLJ t wo component regulatory system plays a major role in inducing nitrogen fixa tion and respiration gene expression in response to the low ambient O-2 con centration of the nodule, Here we report on the mode of action of the FixT protein, a recently identified repressor of nitrogen fixation gene expressi on in S, meliloti, First, we provide evidence that FixT prevents transcript ion of the intermediate key regulatory genes nifA and fixK by counteracting the activity of the FixLJ two-component system under otherwise inducing mi crooxic conditions. Second, we demonstrate that FixT acts as an inhibitor o f the sensor hemoprotein kinase FixL, preventing the production or the accu mulation of its phosphorylated form. FixT is thus a new example of a regula tory protein that blocks signal transduction in two-component systems at th e level of the sensor kinase.