UPTAKE HYDROGENASE (HUP) IN COMMON BEAN (PHASEOLUS-VULGARIS) SYMBIOSES

Strains of Rhizobium forming nitrogen-fixing symbioses with common bea n were systematically examined for the presence of the uptake hydrogen ase (hup) structural genes and expression of uptake hydrogenase (Hup) activity. DNA with homology to the hup structural genes of Bradyrhizob ium japonicum was present in 100 of 248 strains examined. EcoRI fragme nts with molecular sizes of approximately 20.0 and 2.2 kb hybridized w ith an internal SacI fragment, which contains part of both bradyrhizob ial hup structural genes. The DNA with homology to the hup genes was l ocated on pSym of one of the bean rhizobia. Hup activity was observed in bean symbioses with 13 of 30 strains containing D A homologous with the hup structural genes. However, the Hup activity was not sufficien t to eliminate hydrogen evolution from the nodules. Varying the host p lant with two of the Hup+ strains indicated that expression of Hup act ivity was host regulated, as has been reported with soybean, pea, and cowpea strains.