SUMMARY: The formate dehydrogenases of involved in electron transfer from formate to nitrite (Nrf activity: by ) have been identified. No previously undescribed selenoprotein was detected in bacteria grown under conditions optimal for the expression of Nrf activity. The Nrf activities of single mutants defective in either FdhN or FdhH were between 50 and 60% that of the parental strain. A double mutant defective in both FdhN and FdhH retained less than 10 % of the activity of the FdhNFdhH strain. No Nrf activity was detected in a triple mutant defective in FdhN, FdhH and FdhO or in the strain. It is concluded that all three of the known formate dehydrogenases of can contribute to the transfer of electrons from formate to the Nrf pathway. Mutants defective in Nrf activity and cytochrome synthesis were isolated by insertion mutagenesis or identified amongst strains received from the Genetic Stock Center. The mutations were located in at least three regions of the chromosome, including the 92 to 94 minute region which includes the gene encoding FdhH required for formate hydrogenlyase activity. Fine structure mapping by P1 transduction established that the mutations in the region were due to defects in three separable loci, all of which were independent of but close to Clones were isolated from a cosmid library that complemented a deletion extending from into a region essential for Nrf activity. From these clones, plasmids were isolated that complemented only some of the Nrf mutations in the 92 to 94 minute region, confirming the presence of different operons essential for Nrf activity and cytochrome synthesis in this region. Suggested reasons for this genetic complexity include the need for proteins involved in electron transfer from the various formate dehydrogenases to cytochrome for the attachment of the haem group to the apocytochrome and for cytochrome export into the periplasm.


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