The genetic organization of two different 5-nitroimidazole (5-Ni) resistance genes was investigated: and from plasmids plP419 and plP421, respectively. The gene (492 bp) and the 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 and genes previously described and could be derived from a common ancestral gene. An insertion sequence element (IS1170) was identified upstream of the gene. IS1170 is 1604 bp in length and is flanked by imperfect inverted repeats (15 bp). IS 1170 is similar to the 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 . 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 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 carbapenem resistance gene , and the 5-Ni genes and , respectively. There is strong evidence that, as in the case of the gene, the transcription of the four 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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