During a search for genes encoding electron transport proteins from a ATCC 33020 gene bank, a 19.8 kb plasmid, pTF5, which conferred increased sensitivity to the antimicrobial agent metronidazole upon an mutant, was isolated and cloned in The plasmid had an identical restriction enzyme map to a plasmid which has been found in strains isolated from many different parts of the world. The plasmid was present at between two and four copies per genome and contained a region of approximately 5.6 kb which was also found on the chromosome. This region was sequenced and found to have four complete ORFs, which when translated had high percentage amino acid similarity to [3Fe-4S,4Fe-4S] ferredoxins, proteins of the FNR regulator family, prismane-like proteins and the NADH oxidoreductase subunit of a methane monooxygenase. protein analysis using an -derived transcription-translation system indicated that three of the four products (FdxA, PsmA and RedA) were expressed in the heterologous system. Ferredoxins, prismane-like proteins and NADH oxidoreductases are redox-active proteins and it is likely that the proteins on pTF5 represent an electron transport system of as yet unknown function. Surprisingly, although genes for redox-active proteins have been isolated from other bacteria by screening gene banks for increased sensitivity to metronidazole, the region of pTF5 containing the genes for these proteins was not responsible for the increase in metronidazole sensitivity conferred by the plasmid. The region of pTF5 which did confer increased metronidazole sensitivity to an metronidazole-resistant mutant was a 319 bp region of DNA close to the origin of plasmid replication. This region contained no ORFs and was identical to that previously reported for the replicon of a 9.8 kb plasmid, pTFI91.


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