The oxygen-sensing FixLJ represses nitrogen fixation in Rhodopseudomonas palustris in response to oxygen

Biological nitrogen fixation in symbiotic diazotrophs is subject to oxygen regulation by an oxygen-sensing FixLJ two-component system under micro-oxic conditions. However, it remains unclear whether this mechanism is conserved in free-living diazotrophs. In this study, we discovered for the first time that FixLJ strongly inhibits the expression of nifHDK genes that encode molybdenum nitrogenase in response to oxygen. The deletion of fixLJ genes, whose expression was stimulated by oxygen, allowed a free-living photosynthetic diazotroph Rhodopseudomonas palustris to express active nitrogenase and grow diazotrophically even under oxic conditions. The unphosphorylated FixJ protein showed high-affinity binding to the promoter of nitrogenase gene cluster (P_nifH) and strongly repressed the nitrogenase expression in response to oxygen. The transcriptional repression of nifHDK by FixJ reveals a new regulatory role for the FixLJ system. In addition, transcriptome analysis suggested that the FixLJ regulatory system also plays a role in the energy metabolism of R. palustris, probably through FixK regulation. This newly identified mechanism is speculated to allow R. palustris to rapidly shut down the synthesis of nitrogenase when exposed to oxygen, avoiding the build-up of nitrogenase with impaired activity due to the lack of protection from oxygen damage.