title: Regulatory changes in the formation of chromosomal dihydrofolate reductase causing resistance to trimethoprim.
abstract: High resistance to trimethoprim mediated by the several hundredfold overproduction of the drug target enzyme, dihyrofolate reductase, in a clinically isolated Escherichia coli strain, 1810, was cloned onto several vector plasmids and seemed to be comprised of a single dihydrofolate reductase gene, which by DNA-DNA hybridization and restriction enzyme digestion mapping was very similar to the corresponding gene of E. coli K-12. Determination of mRNA formation in the originally isolated resistant strain and strains with cloned trimethoprim resistance determinant demonstrated an about 15-fold increase in production of dihydrofolate reductase mRNA compared with that in E. coli K-12. This was explained by the occurrence of a promoter up mutation in the resistant isolate accompanied by changes in the restriction enzyme digestion pattern found by comparison with the corresponding pattern from E. coli K-12.
mesh_terms: D001483:Base Sequence; D002876:Chromosomes, Bacterial / Q000502:physiology*; D004262:DNA Restriction Enzymes; D004352:Drug Resistance, Microbial; D004926:Escherichia coli / Q000187:drug effects / Q000201:enzymology / Q000235:genetics*; D005796:Genes; D005798:Genes, Bacterial; D005809:Genes, Regulator; D007700:Kinetics; D010957:Plasmids; D012333:RNA, Messenger / Q000235:genetics; D013045:Species Specificity; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics*; D014295:Trimethoprim / Q000633:toxicity*
chemical_list: D012333:RNA, Messenger; D014295:Trimethoprim; D013762:Tetrahydrofolate Dehydrogenase; D004262:DNA Restriction Enzymes