Updated project metadata. Hydrogenases catalyze the simple yet importantredox reaction between protons and electrons and H2, thus, mediate interactions between microorganisms and their chemical environment. A mutation in M. huakuii hypE gene was generated by homologous recombination. The hypE mutant grew a little faster than its parental 7653R. Real-time quantitative PCR showed that the hypE gene expression is significantly upregulated in all the detected stages of nodule development. The wild type contained normal spherical shape of the nodules, while the hypE mutant elicited small-size nodules. Plants inoculated with mutant HKhypE showed a significantly more than 47% decrease in acetylene reduction activity compared to the wild-type 7653R. Proteomics analysis showed seven proteins were up-regulated and eighty-two proteins were down-regulated in hypE triple mutant bacteroids.