HYDROGENASE IN BRADYRHIZOBIUM-JAPONICUM - GENETICS, REGULATION AND EFFECT ON PLANT-GROWTH

A region of 15 kbp DNA located on the chromosome of Bradyrhizobium jap onicum 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 O 2, and regulation of hydrogenase expression in free-living conditions, have been identified in this region. A region in front of the hydroge nase structural genes is necessary for transcriptional regulation of h ydrogenase expression by O2, H-2 and Ni2+. In addition, the N2 fixatio n system and the tertiary structure of the chromosome seem to be invol ved in the expression of the hydrogenase genes. The effect of legume i noculation with Hup+ rhizobia has been evaluated. The possible benefit s 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 nit rogenase reaction, are discussed. The data indicate that a H-2-uptake system is beneficial in soybean nodules, but host plant and environmen tal factors may interfere with the effects of H-2 cycling in the plant .