"Active Site-Directed Double Mutants of Dihydrofolate Reductase."

According to the authors' abstract of an article published in Cancer Research, "Variants of dihydrofolate reductase (DHFR), which confer resistance to antifolates, are used as dominant selectable markers in vitro and in vivo and may be useful in the context of gene therapy. To identify improved mutant human DHFRs with increased catalytic efficiency and decreased binding to methotrexate, we constructed by site-directed mutagenesis four variants with substitutions at both Leu(22) and Phe(31) (i.e., Phe(22)-Ser(31), Tyr(22)-Ser(31), Phe(22)-Gly(31), and Tyr(22)-Gly(31)). Antifolate resistance has been observed previously when individual changes are made at these active-site residues. ...

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