Rhizobium japonicum mutants that are hypersensitive to repression of H2 uptake by oxygen

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Abstract
The synthesis of an H2 oxidation system in free-living R. japonicum wild-type strain SR is repressed by O2. Maximal H2 uptake rates were obtained in strain SR after derepression in 11 .mu.M or less dissolved O2. O2 levels above 45 .mu.M completely repressed H2 uptake in strain SR. Five R. japonicum mutant strains that are hypersensitive to repression of H2 oxidation by O2 derived from strain SR. The mutants were obtained by screening H2 uptake-negative mutants that retained the ability to oxidize H2 as bacteriods from soybean nodules. As bacteroids, the 5 mutant strains were capable of H2 oxidation rates comparable to that of the wild type. The mutants did not take up H2 when derepressed in 22 .mu.M dissolved O2, whereas strain SR had substantial activity at this O2 concentration. The O2 repression of H2 uptake in the wild-type and 2 mutant strains, SR174 and SR200, was rapid and was similar to the effect of inhibiting synthesis of H2 uptake system components with rifampin. None of the mutant strains was able to oxidize H2 when the artificial electron acceptors methylene blue or phenazine methosulfate were provided. The mutant strains were not sensitive to killing by O2, they took up O2 at rates similar to strain SR and they did not produce an H2 uptake system that was O2 labile. cAMP levels were comparable in strain SR and the 5 mutant strains after subjection of the cultures to the derepression conditions.

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