The photoproduction of H2 and NH+4 fixed from N2 by a derepressed mutant of Rhodospirillum rubrum

A mutant of Rhodospirillum rubrum has been isolated, after mutagenesis with nitrosoguanidine, which is characterized by its inability to grow in the light on malate-minimal media with exogenous ammonia or alanine, poor growth on glutamine and vigorous growth on glutamate. This mutant produces low levels of a key NH⁺₄ assimilation enzyme, glutamate synthase (NADPH-dependent). It also exhibits significant derepression of nitrogenase biosynthesis in the presence of ammonia or alanine, being 15% derepressed for the former and about 70% derepressed for the latter.Some of this mutant's fixed N₂ is excreted into the medium as NH⁺₄ (1 μmol NH⁺₄ per mg cell protein in 50 h). Nitrogenase-mediated H₂ production by this strain is considerable (42 μmol H₂ per mg cell protein in 50 h), approximately twice that of the wild type assayed under similar conditions.These results demonstrate that genetic alteration of the photosynthetic N₂-fixer's NH⁺₄ assimilation system disrupts the tight coupling of N₂ fixation and NH⁺₄ assimilation normally observed in these organisms, enabling photochemical conversion steps to be utilized for the photoproduction of NH⁺₄ and H₂.