Backbone and side‑chain 1H, 13C and 15N resonance assignments and secondary structure determination of the rhizobial FixJ

The symbiotic nitrogen-fixing bacterium Bradyrhizobium japonicum (B.japonicum) enables high soybean yields with little or no nitrogen fertiliser. A two component regulatory system comprising FixL, a histidine kinase with O₂-sensing activity, and FixJ, a response regulator, controls the expression of genes involved in nitrogen fixation, such as fixK and nifA. Only under anaerobic conditions, the monophosphate group is transferred from FixL to the N-terminal receiver domain of FixJ (FixJ_N), which eventually promote the association of the C-terminal effector domain (FixJ_C) to the promoter regions of the nitrogen-fixation-related genes. Structural biological analyses carried out so far for rhizobial FixJ molecules have proposed a solution structure for FixJ that differs from the crystal structures, in which the two domains are extended. To understand the FixJ activation caused by phosphorylation of the N-terminal domain, which presumably regulates through the interactions between FixJ_N and FixJ_C, here we have performed backbone and sidechain resonance assignments of the unphosphorylated state of B. japonicum FixJ.