Hydrogenase in Bradyrhizobium japonicum: genetics, regulation and effect on plant growth.

A region of 15 kbp DNA located on the chromosome of Bradyrhizobium japonicum is essential for a Hup(+) phenotype and contains the hydrogenase structural genes. During the last few years, other genes with various functions related to hydrogenase oxidation, such as Ni(2+) incorporation into the hydrogenase enzyme, electron transport from hydrogenase to O2, and regulation of hydrogenase expression in free-living conditions, have been identified in this region. A region in front of the hydrogenase structural genes is necessary for transcriptional regulation of hydrogenase expression by O2, H2 and Ni(2+). In addition, the N2 fixation system and the tertiary structure of the chromosome seem to be involved in the expression of the hydrogenase genes. The effect of legume inoculation with Hup(+) rhizobia has been evaluated. The possible benefits of a Hup(+) phenotype, such as the regeneration of chemical energy in the form of ATP or reductants, and the removal of O2 and H2 from the active site of the nitrogenase enzyme where they might inhibit the nitrogenase reaction, are discussed. The data indicate that a H2-uptake system is beneficial in soybean nodules, but host plant and environmental factors may interfere with the effects of H2 cycling in the plant.