Energy coupling efficiency of symbiotic nitrogen fixation

Expression of hydrogenase activity in free-living Rhizobium japonicum

A medium is described on which selected Rhizobium japonicum strains express hydrogenase (H(2) uptake) activity under free-living conditions. Low concentrations of carbon substrates, decreased oxygen tension, and the quantity of combined nitrogen in the medium were major factors influencing hydrogenase expression. Hydrogenase activity was dependent upon a preincubation period in the presence of H(2) under conditions such that the cells did not exhibit nitrogenase activity. H(2) uptake rates were easily measured amperometrically in aerobically or anaerobically prepared suspensions from free-living cultures. Six R. japonicum strains that formed nodules with the ability to utilize H(2) oxidized this gas when grown in free-living cultures. In comparison six randomly chosen strains forming nodules that lost H(2) in air either showed no or low capacity to take up H(2) under free-living conditions. The reduction of triphenyltetrazolium chloride in an agar medium was used to detect strains capable of oxidizing H(2). This method has enabled us to isolate a spontaneous R. japonicum mutant strain that has lost the ability to utilize H(2). This mutant strain forms nodules that evolve H(2) but other symbiotic characteristics appear normal. This strain will be useful in evaluating the importance of the hydrogenase system in the nitrogen-fixing process of legumes.

Revertible hydrogen uptake-deficient mutants of Rhizobium japonicum

We have developed mutants of Rhizobium japonicum which are deficient in H2 uptake capacity (Hup-) and which spontaneously revert to the parent type at a frequency consistent with that of a single-point mutation (ca. 1.0 x 10(-09)). The mutagenesis by nitrous acid and the selection of the Hup- phenotype by using penicillin and chemolithotrophy as enrichment for chemolithotrophy-deficient strains are described. Two mutants retain low but reproducible levels of ribulose bisphosphate-dependent CO2 fixation when grown on a low-carbon medium under an atmosphere of 1% O2, 4% H2, 5% CO2, and 90% N2. Neither O2 nor the artificial electron acceptors phenazine methosulfate or methylene blue supported detectable H2 uptake by the free-living Hup- mutants or by their bacteroids. Plant growth experiments under bacteriologically controlled conditions were conducted to assess the mutants' performance as inocula for soybean plants. Plants inoculated with Hup- strains had lower dry weights and contained less total N than did plants inoculated with the parent Hup+ strain. Use of either the Hup- mutants or the Hup+ parent strain as inocula, however, did not significantly affect the acetylene-reducing activity or the fresh weight of nodules. These results, obtained with apparently isogenic lines of H2 uptake-deficient R. japonicum, provide strong support for a beneficial role of the H2 uptake phenotype in legume symbiosis.

Hydrogenase genes

For a variety of reasons, including the potential industrial applications of hydrogenase, we are interested in the isolation and analysis of hydrogenase genes. In a program focusing on the hydrogen bacterium A. eutrophus H1 and E. coli, we have developed a preliminary concept of the interaction of hydrogenase in cellular metabolism, constructed mutants deficient in hydrogenase activity, and begun the isolation of hydrogenase genes utilizing the technology allowing the in vitro manipulation of DNA. We hope to pursue this project to its ultimate goal: the analysis of the molecular mechanisms involved in the control of expression of these genes and the development of the ability to manipulate the production of hydrogenase.

Regulation of hydrogenase in Rhizobium japonicum

Factors that regulate the expression of an H2 uptake system in free-living cultures of Rhizobium japonicum have been investigated. Rapid rates of H2 uptake by R. japonicum were obtained by incubation of cell suspensions in a Mg-phosphate buffer under a gas phase of 86.7% N2, 8.3% H2, 4.2% CO2, and 0.8% O2. Cultures incubated under conditions comparable with those above, with the exception that Ar replaced H2, showed no hydrogenase activity. When H2 was removed after initiation of hydrogenase derepression, further increase in hydrogenase activity ceased. Nitrogenase activity was not essential for expression of hydrogenase activity. All usable carbon substrates tested repressed hydrogenase formation, but none of them inhibited hydrogenase activity. No effect on hydrogenase formation was observed from the addition of KNO3 or NH4Cl at 10 mM. Oxygen repressed hydrogenase formation, but did not inhibit activity of the enzyme in whole cells. The addition of rifampin or chloramphenicol to derepressed cultures resulted in inhibition of enzyme formation similar to that observed by O2 repression. The removal of CO2 during derepression caused a decrease in the rate of hydrogenase formation. No direct effect of CO2 on hydrogenase activity was observed.