Novel streptomycin and spectinomycin resistance gene as a gene cassette within a class 1 integron isolated from Escherichia coli. Antimicrob. Agents Chemother

The aadA genes, encoding resistance to streptomycin and spectinomycin, have been found as gene cassettes in different gram-negative and gram-positive bacterial species. The present study has revealed the sequence of a new gene, aadA5, integrated as a gene cassette together with the trimethoprim resistance gene dfr7 in a class 1 integron. The integron was located on a plasmid and was identified in a pathogenic porcine Escherichia coli isolate. Hollingshead and Vapnek (12) were the first to describe aadA [also called ant(30)21], which encodes an adenylylation enzyme that modifies streptomycin and spectinomycin. Sund-ström et al. (20) sequenced aadA1 as a gene cassette and the class 1 integron from Tn21 (GenBank accession no. 12870). They found that the cassette could be translated, and they recognized the target of a site-specific recombination site (59-bp element). The gene cassettes identified so far have been in the same orientation, and it is possible that the 59-bp ele-ment has a role in determination of the orientation when gen