title: Azorhizobium caulinodans pyruvate dehydrogenase activity is dispensable for aerobic but required for microaerobic growth.
abstract: Azorhizobium caulinodans mutant 62004 carries a null allele of pdhB, encoding the E1beta subunit of pyruvate dehydrogenase, which converts pyruvate to acetyl-CoA. This pdhB mutant completely lacks pyruvate oxidation activities yet grows aerobically on C(4) 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 N(2) in aerobic culture, but is unable to fix N(2) in symbiosis with host Sesbania rostrata plants and cannot grow microaerobically. In culture, A. caulinodans wild-type can use acetate, beta-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.
authors.affiliation: Department of MCD Biology, Sinsheimer Laboratories, University of California, Santa Cruz, CA 95064, USA1.
authors.forename: David C
authors.initials: DC
authors.lastname: Pauling
authors.affiliation: Department of MCD Biology, Sinsheimer Laboratories, University of California, Santa Cruz, CA 95064, USA1.
authors.forename: Jerome P
authors.initials: JP
authors.lastname: Lapointe
authors.affiliation: Department of MCD Biology, Sinsheimer Laboratories, University of California, Santa Cruz, CA 95064, USA1.
authors.forename: Carolyn M
authors.initials: CM
authors.lastname: Paris
authors.affiliation: Department of MCD Biology, Sinsheimer Laboratories, University of California, Santa Cruz, CA 95064, USA1.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Ludwig
mesh_terms: D000105:Acetyl Coenzyme A / Q000378:metabolism; D000332:Aerobiosis; D020599:Azorhizobium caulinodans / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development*; D003001:Cloning, Molecular; D003470:Culture Media; D005816:Genetic Complementation Test; D006885:Hydroxybutyrates / Q000378:metabolism; D008969:Molecular Sequence Data; D009154:Mutation; D009243:NAD / Q000378:metabolism; D009586:Nitrogen Fixation; D010084:Oxidation-Reduction; D011768:Pyruvate Dehydrogenase Complex / Q000235:genetics / Q000378:metabolism*; D011773:Pyruvates / Q000378:metabolism; D013386:Succinates / Q000378:metabolism
chemical_list: D003470:Culture Media; D006885:Hydroxybutyrates; D011768:Pyruvate Dehydrogenase Complex; D011773:Pyruvates; D013386:Succinates; D009243:NAD; D000105:Acetyl Coenzyme A