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Abstract
The genetic organization of two different 5-nitroimidazole (5-Ni) resistance genes was investigated: nimC and nimD from Bacteroides plasmids plP419 and plP421, respectively. The nimC gene (492 bp) and the nimD gene (495 bp) directed the synthesis of polypeptides with deduced molecular masses of 18·37 kDa and 18·48 kDa, respectively. The predicted proteins showed 67-83% identity and 78-91% similarity with the products of two other nimA and nimB genes previously described and could be derived from a common ancestral gene. An insertion sequence element (IS1170) was identified upstream of the nimC gene. IS1170 is 1604 bp in length and is flanked by imperfect inverted repeats (15 bp). IS 1170 is similar to the Bacteroides insertion sequence element IS942 with an identity of 70% at the nucleotide level. The single copy of IS 1170 present on plasmid plP419 is integrated 24 bp upstream of the initiation codon of nimC. Similar genetic organization was found on plasmid plP421. One copy of another insertion sequence (IS1169) was found 4 bp upstream of the first ATG codon of the nimD gene. This element (1325 bp) shows a strong homology at the nucleotide level (70% identity) with IS 1186 and IS 1168 found to be associated with the Bacteroides carbapenem resistance gene cfiA, and the 5-Nir genes nifiA and nimB, respectively. There is strong evidence that, as in the case of the cfiA gene, the transcription of the four nim genes so far studied is directed by outward-oriented promoters, carried on the right ends of the different insertion sequence elements.
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