CONICET | Buscador de Institutos y Recursos Humanos

Bradyrhizobium diazoefficiens is a soil bacterium able to establish nitrogen-fixing symbiosis with soybean plants. In free-living this rhizobium utilizes different nitrogen sources like aminoacids, nitrate and ammonium. Our group focuses on the analysis of possible regulators involved in nitrogen assimilation in B.diazoefficiens, since it is known that in nitrogen-starved cultures several stages of the symbiotic process are stimulated (1). One of these regulators is NtrBC, a two-component system (TCS) that senses the intracellular variations of C/N ratio and controls the expression of some genes implicated in ammonium assimilation. Another TCS is NtrYX, located immediately downstream of the nifR3-ntrB-ntrC operon. This system was reported in other α-proteobacteria in which it has a regulatory role in diverse processes such as, nitrogen metabolism (2), symbiotic nodulation (3), motility (3) and acting as a redox sensor in response to low-oxygen conditions (4). Particularly, in this work we show the characterization of the NtrYX system of B. diazoefficiens USDA 110.Initially, we generated a non-polar mutant of the sensor protein NtrY of the system (named LP4489) and observed that its growth under aerobic conditions with ammonium or nitrate as the sole nitrogen source, is not significantly different compared to wild-type strain. In addition, we analysed other phenotypes like exopolysaccharide production, motility, biofilm formation and symbiotic nitrogen fixation, finding that all of them were affected by the ntrY mutation.In conclusion, the results obtained in our studies suggest that ntrY is a pleiotropic gene in this rhizobium, as it is involved in all the other processes mentioned above, but it is not required to assimilate ammonium or nitrate in aerobic cultures. Regarding the regulatory protein NtrX, we have not accomplished to generate the mutant yet, presumably because it is lethal, as it occurs in other bacteria.