Abstract
Nitrogen fixation in symbiotic rhizobia is subject to multiple levels of gene regulation. In Sinorhizobium meliloti, the alfalfa symbiont, the FixLJ two-component regulatory system plays a major role in inducing nitrogen fixation and respiration gene expression in response to the low ambient O(2) concentration of the nodule. Here we report on the mode of action of the FixT protein, a recently identified repressor of nitrogen fixation gene expression in S. meliloti. First, we provide evidence that FixT prevents transcription of the intermediate key regulatory genes nifA and fixK by counteracting the activity of the FixLJ two-component system under otherwise inducing microoxic conditions. Second, we demonstrate that FixT acts as an inhibitor of the sensor hemoprotein kinase FixL, preventing the production or the accumulation of its phosphorylated form. FixT is thus a new example of a regulatory protein that blocks signal transduction in two-component systems at the level of the sensor kinase.
Highlights
In legumes, nitrogen fixation takes place in a specialized organ, the nodule, that rhizobia elicit on the roots of their host plants
Positive control is exerted by a two-component regulatory system, FixLJ, that senses the microoxic conditions that prevail inside the nodule [1, 2]
We further demonstrate that FixT behaves as an inhibitor of the FixL sensor kinase, preventing synthesis or accumulation of its phosphorylated form
Summary
Nitrogen fixation takes place in a specialized organ, the nodule, that rhizobia elicit on the roots of their host plants. Phosphorylation of FixJ dramatically enhances its affinity for the promoters of two intermediate regulatory genes, nifA and fixK, that control expression of genes involved in the biosynthesis of nitrogenase and of an oxidase complex with high affinity for oxygen, respectively [5,6,7]. We provide evidence that the FixT protein prevents transcription of nifA and fixK by counteracting the activity of the master FixLJ two-component system under microoxic conditions. We further demonstrate that FixT behaves as an inhibitor of the FixL sensor kinase, preventing synthesis or accumulation of its phosphorylated form. Biological implications of this finding are discussed with respect to the regulation of symbiotic nitrogen fixation as well as in terms of signal processing by two-component regulatory systems in general
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
