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 resistanc e was found inserted in a cassette-like manner in the recombinationall y active locus, the integron, borne on a transposon Tn21-like element. The dhfrXII cassette is distinct from those cassettes earlier observe d in integrons and was found here upstream of two similarly inserted c assettes. The second one carried the new unidentified orjF, which is 8 5% identical to the orfD cassette in R46. The third cassette contained the aadA2 gene mediating spectinomycin resistance. The plasmid carryi ng this Tn21-like element was originally isolated from a trimethoprim- resistant urinary tract pathogen, Escherichia coli, from Turku City Ho spital, Turku, Finland. By colony hybridization and polymerase chain r eaction, this group of three cassettes, including dhfrXII, was detecte d in four additional E. coli strains of similar origin and in four Shi gella strains isolated in Finland but originating from Asia. The dihyd rofolate reductase produced from dhfrXII showed an unusual drug resist ance in that 50% of the enzymatic activity remained at a trimethoprim concentration of 1 mM.