Amino-acid sequence of dihydrofolate reductase from a methotrexate-resistant mutant of Streptococcus faecium and identification of methionine residues at the inhibitor binding site.

The amino-acid sequence of dihydrofolate reductase (7,8-dihydrofolate:NADP(+) oxidoreductase, EC 1.5.1.4) from S. faecium var Durans strain A is reported, and methionine residues 28 and 50 are shown to be protected by the inhibitor aminopterin from carboxymethylation by iodoacetate which occurs in absence of the inhibitor. Comparison of the sequence with that of the Escherichia coli reductase reveals two domains of considerable homology, one (the N-terminal region) presumably concerned with dihydrofolate and inhibitor binding and the other with dinucleotide binding. No significant sequence homology was found between larger dehydrogenases and the dihydrofolate reductases, which must, therefore, have evolved from a different ancestral protein.