紫云英根瘤菌及巴西固氮螺菌的氢酶表达特征

紫云英根瘤菌氢酶表达依赖于H_2并受碳底物和高O_2浓度的阻遏及cAMP的显著促进。整体细胞的吸氢活性对O_2不敏感,受碘乙酸(50mmol L~(-1))的强烈抑制。少数氧化还原电位为正值的人工电子受体可支持吸氢活性。与紫云英根瘤菌不同,巴西固氮螺菌氢酶表达并不依赖于H_2,受碳底物阻遏及cAMP促进的效应均不显著,而对O_2敏感。整体细胞吸氢活性受碘乙酸的抑制作用不明显。无论正、负值氧化还原电位人工电子受体均可支持吸氢活性。在经饥饿的静止细胞中,H_2可支持固氮活性并增强固氮酶对O_2的耐受能力。

Expression and Activity of Hydrogenase Systems in Rhizobium astragalus and Azospirillum brasilense

H_2-uptake systems of Rhizobium astragalus and Azospirillura brasilense were compared. The expression of hydrogenase in free-living cultures of R. astragalus strain 109 was dependent on H_2 and enhanced by the addition of cAMP (1.0 mmol L~(-1)), while high levels of O_2 and C substrates repressed the formation of the enzyme. H_2-uptake activity in vivo was insensitive to O_2 and inhibited by iodoacetic acid (50 mmol L~(-1)). A few of the positive-potential electron acceptors, such as methylene blue, ferricyanide and 2,6-dichlorophenolindophenol, could support the H_2-uptake activity. H_2-uptake system was also studied both in N_2-fixing and NH_4~ -grown batch cultures of A. brasilense Abw. Some major differences in hydrogenase expression in this bacterium to R. astragalus were demonstrated, i.e., the requirement of microaerobiosis, independence from H_2 or nitrogenase induction, and insensitivity to C substrates. The H_2-uptake activity in intact cells of A. brasilense was very sensitive to O_2, and was partially inhibited by iodoacetic acid (50 mmol L~(-1)). Either positive-or negative-potential electron acceptors could support the Ha-uptake activity. H_2-dependent acetylene reduction was only found in resting cells depleted of endogenous substrates, In such N_2 fixing cells the oxyhydrogen reaction could help to reduce the O_2 tension, so that the nitrogenase activity was better protected against O_2 inacti vation.