Identification of the Hydrogen Uptake Gene Cluster for Chemolithoautotrophic Growth and Symbiosis Hydrogen Uptake in Bradyrhizobium Diazoefficiens

The hydrogen uptake (Hup) system of Bradyrhizobium diazoefficiens recycles the H-2 released by nitrogenase in soybean nodule symbiosis, and is responsible for H-2-dependent chemolithoautotrophic growth. The strain USDA110 has two hup gene clusters located outside (locus I) and inside (locus II) a symbiosis island. Bacterial growth under H-2-dependent chemolithoautotrophic conditions was markedly weaker and H-2 production by soybean nodules was markedly stronger for the mutant of hup locus I (Delta hupS(1)L(1)) than for the mutant of hup locus II (Delta hupS(2)L(2)). These results indicate that locus I is primarily responsible for Hup activity.