Cloning of dihydrofolate reductase gene of Escherichia coli K12

Resistant strains for trimethoprim, a potent inhibitor of dihydrofolate reductase, were obtained by transforming the ligated products of Escherichia coli K12 DNA and plasmid pBR 322 BamH I fragments. The strains carry a 13.6 kbp plasmid, pTP 1, which contains the trimethoprim- and ampicillin-resistance determinant genes. The trimethoprim-resistance determinant gene was estimated to consist of more than 500 nucleotides and less than 1,500 nucleotides and was restricted b