World Journal of Microbiology and Biotechnology vol:9 pages:615-624
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 Ni2+ 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, H-2 and Ni2+. 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 H-2 from the active site of the nitrogenase enzyme where they might inhibit the nitrogenase reaction, are discussed. The data indicate that a H-2-uptake system is beneficial in soybean nodules, but host plant and environmental factors may interfere with the effects of H-2 cycling in the plant.