A novel dihydrofolate reductase cassette inserted in an integron borne on a Tn21-like element

In this study, a 498-bp dhfrXII gene coding for trimethoprim resistance was found inserted in a cassette-like manner in the recombinationally active locus, the integron, borne on a transposon Tn21-like element. The dhfrXII cassette is distinct from those cassettes earlier observed in integrons and was found here upstream of two similarly inserted cassettes. The second one carried the new unidentified orfF, which is 85% identical to the orfD cassette in R46. The third cassette contained the aadA2 gene mediating spectinomycin resistance. The plasmid carrying this Tn21-like element was originally isolated from a trimethoprim-resistant urinary tract pathogen, Escherichia coli, from Turku City Hospital, Turku, Finland. By colony hybridization and polymerase chain reaction, this group of three cassettes, including dhfrXII, was detected in four additional E. coli strains of similar origin and in four Shigella strains isolated in Finland but originating from Asia. The dihydrofolate reductase produced from dhfrXII showed an unusual drug resistance in that 50% of the enzymatic activity remained at a trimethoprim concentration of 1 mM.