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The amino acid sequence of a trimethoprim-resistant dihydrofolate reductase (EC
1.5.1.3) specified by the R-plasmid R67 is described. The sequence was deduced
from automatic and manual sequence analysis of the intact protein, the fragments
produced by cyanogen bromide cleavage, and peptides derived from the largest
cyanogen bromide fragment by digestion with trypsin, Staphylococcus aureus V8
proteus, chymotrypsin, and Lysobacter enzymogenes alpha-lytic protease. The
complete sequence comprises 78 residues in a single polypeptide chain of
molecular weight 8444. No evidence of heterogeneity was obtained, indicating
that all subunits of the native enzyme are identical. Comparison of the sequence
with that of all known dihydrofolate reductases shows no significant sequence
homology.
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