title: Monitoring of plasmid-encoded, trimethoprim-resistant dihydrofolate reductase genes: detection of a new resistant enzyme.
abstract: Using-gene-specific radiolabeled probe DNAs, we analyzed 42 clinical bacterial isolates with high-level trimethoprim (Tp) resistance for the presence of a type I or a type II plasmid-specified dihydrofolate reductase (DHFR) gene. Plasmid DNA from 17 strains harbored a type I DHFR, whereas 11 isolates contained plasmids that harbored a type II DHFR structural gene. The plasmid DNAs from five strains appeared to hybridize with both type I and type II DHFR probe DNAs. In addition, eight isolates had type I resistance determinants integrated into the chromosomes, presumably on transposon 7 (Tn7). Among the strains analyzed in this survey, none of the chromosomally located, Tp-insensitive reductases were of the type II class. Both the plasmid and chromosomal DNAs of one isolate showed no homology with either the type I or type II DHFR probe DNA. The plasmid harbored by this strain encoded a "new" Tp-resistant enzyme that differed significantly, both in molecular weight and with respect to trimethoprim and methotrexate inhibition kinetics, from the previously characterized plasmid-associated dihydrofolate reductases.
mesh_terms: D001419:Bacteria / Q000201:enzymology*; D004269:DNA, Bacterial / Q000032:analysis; D005796:Genes; D011815:R Factors; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics*; D014295:Trimethoprim / Q000494:pharmacology*
chemical_list: D004269:DNA, Bacterial; D014295:Trimethoprim; D013762:Tetrahydrofolate Dehydrogenase