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Cape Town, Private Bag Rondebosch, Cape Town 7700, South Africa During a search for genes encoding electron transport proteins from a Thiobacillus ferrooxidans ATCC 33020 gene bank, a 19.8 kb plasmid, pTF5, which conferred increased sensitivity to the antimicrobial agent metronidazole upon an Escherichia coli mutant, was isolated and cloned in Em coli. The plasmid had an identical restriction enzyme map to a plasmid which has been found in 1. ferrooxidans strains isolated from many different pa-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 t o [3Fe+S,4Fe-QS] ferredoxins, proteins of the FNR regulator family, prismanelike proteins and the NADH oxidoreductase subunit of a methane monooxygenase. In witm protein analysis using an E. coli-derived transcriptiontranslation system indicated that t h m 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 Lm coli 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 Tm fkmxidans plasmid, pTF191.University Of