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.