The nitrate‐sensing N as ST system regulates nitrous oxide reductase and periplasmic nitrate reductase in B radyrhizobium japonicum

Summary The soybean endosymbiont B radyrhizobium japonicum is able to scavenge the greenhouse gas N 2 O through the N 2 O reductase ( N os). In previous research, N 2 O emission from soybean rhizosphere was mitigated by B . japonicum   N os ++ strains (mutants with increased N os activity). Here, we report the mechanism underlying the N os ++ phenotype. Comparative analysis of N os ++ mutant genomes showed that mutation of bll4572 resulted in N os ++ phenotype. bll4572 encodes NasS , the nitrate ( NO 3 − )‐sensor of the two‐component NasST regulatory system. Transcriptional analyses of nos Z (encoding N os) and other genes from the denitrification process in nas S and nas ST mutants showed that, in the absence of NO 3 − , nasS mutation induces nos Z and nap (periplasmic nitrate reductase) via nas T . NO 3 − addition dissociated the NasS ‐ NasT complex in vitro , suggesting the release of the activator NasT . Disruption of nas T led to a marked decrease in nos Z and nap transcription in cells incubated in the presence of NO 3 − . Thus, although NasST is known to regulate the NO 3 − ‐mediated response of NO 3 − assimilation genes in bacteria, our results show that NasST regulates the NO 3 − ‐mediated response of nos Z and nap E genes, from the dissimilatory denitrification pathway, in B . japonicum .