K12 mutants lacking phenazine-methosulphate-linked formate dehydrogenase (FDH-PMS) activity, but still capable of producing normal levels of benzyl-viologen-linked formate dehydrogenase (FDH-BV) and nitrate reductase activities, have been isolated following P1 localized mutagenesis. The relevant mutations mapped with the same cotransduction frequency close to the gene, at 88 min on the chromosome. They were further subdivided into two classes. Class I consisted of six mutants which synthesized an inactive FDH-PMS protein with the same subunit composition as the wild-type enzyme. In contrast, class II contained four mutants totally devoid of this antigen. Construction of merodiploid strains harbouring various combinations of the mutated alleles, on the episome and on the chromosome, led to the restoration of FDH-PMS activity by complementation of the products encoded by the respective wild-type alleles. Difference spectroscopy suggested that both and mutants contained normal amounts of the cytochrome associated with FDH-PMS although the cytochrome had lost its capacity for formate-dependent reduction.


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