Bradyrhizobium ottawaense efficiently reduces nitrous oxide through high nosZ gene expression

N 2 O is an important greenhouse gas influencing global warming, and agricultural land is the predominant (anthropogenic) source of N 2 O emissions. Here, we report the high N 2 O-reducing activity of Bradyrhizobium ottawaense , suggesting the potential for efficiently mitigating N 2 O emission from agricultural lands. Among the 15 B. ottawaense isolates examined, the N 2 O-reducing activities of most (13) strains were approximately five-fold higher than that of Bradyrhizobium diazoefficiens USDA110 T under anaerobic conditions. This robust N 2 O-reducing activity of B. ottawaense was confirmed by N 2 O reductase (NosZ) protein levels and by mitigation of N 2 O emitted by nodule decomposition in laboratory system. While the NosZ of B. ottawaense and B. diazoefficiens showed high homology, nosZ gene expression in B . ottawaense was over 150-fold higher than that in B. diazoefficiens USDA110 T , suggesting the high N 2 O-reducing activity of B. ottawaense is achieved by high nos expression. Furthermore, we examined the nos operon transcription start sites and found that, unlike B. diazoefficiens , B . ottawaense has two transcription start sites under N 2 O-respiring conditions, which may contribute to the high nosZ expression . Our study indicates the potential of B. ottawaense for effective N 2 O reduction and unique regulation of nos gene expression towards the high performance of N 2 O mitigation in the soil.