1887

Abstract

thermolabile point mutants unable to fix N at 42 °C were isolated and mapped to three, unlinked loci; from complementation tests, several mutants were assigned to the locus. Of these, two independent mutants carried missense substitutions in the product electron-transferring flavoprotein N (ETF) -subunit. Both thermolabile missense variants Y238H and D229G mapped to the ETF interdomain linker. Unlinked thermostable suppressors of these two missense mutants were identified and mapped to the gene, encoding dihydrolipoamide dehydrogenase (LpDH), immediately distal to the genes, which collectively encode the pyruvate dehydrogenase (PDH) complex. These two suppressor alleles encoded LpDH NAD-binding domain missense mutants G187S and E210G. Crude cell extracts of these double mutants showed 60–70 % of the wild-type PDH activity; neither double mutant strain exhibited any growth phenotype at the restrictive or the permissive temperature. The genetic interaction between two combinations of and missense alleles implies a physical interaction of their respective products, LpDH and ETF. Presumably, this interaction electrochemically couples LpDH as the electron donor to ETF as the electron acceptor, allowing PDH complex activity (pyruvate oxidation) to drive soluble electron transport via ETF to N, which acts as the terminal electron acceptor. If so, then, the PDH complex activity sustains N fixation both as the driving force for oxidative phosphorylation and as the metabolic electron donor.

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2004-01-01
2019-10-18
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