Regulatory changes in the formation of chromosomal dihydrofolate reductase causing resistance to trimethoprim

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.