Azorhizobium caulinodans pyruvate dehydrogenase activity is dispensable for aerobic but required for microaerobic growthb

摘要

Azorhizobium caulinodans mutant 62004 carries a null allele of pdhB, encoding the E1β subunit of pyruvate dehydrogenase, which converts pyruvate to acetyl-CoA. This pdhB mutant completely lacks pyruvate oxidation activities yet grows aerobically on C4 dicarboxylates (succinate, L-malate) as sole energy source, albeit slowly, and displays pleiotropic growth defects consistent with physiological acetyl-CoA limitation. Temperature-sensitive (ts), conditional-lethal derivatives of the pdhB mutant lack (methyl)malonate semialdehyde dehydrogenase activity, which thus also allows L-malate conversion to acetyl-CoA. The pdhB mutant remains able to fix N2 in aerobic culture, but is unable to fix N2 in symbiosis with host Sesbania rostrata plants and cannot grow microaerobically. In culture, A. caulinodans wild-type can use acetate, β-D-hydroxybutyrate and nicotinate – all direct precursors of acetyl-CoA – as sole C and energy source for aerobic, but not microaerobic growth. Paradoxically, acetyl-CoA is thus a required intermediate for microaerobic oxidative energy transduction while not itself oxidized. Accordingly, A. caulinodans energy transduction under aerobic and microaerobic conditions is qualitatively different.