Dihydrofolate reductase from Lactobacillus casei: N-terminal sequence and comparison with the substrate binding region of other reductases.

We report the N-terminal amino acid sequence of dihydrofolate reductase from a methotrexate-resistant strain of $\text{Lactobacillus casei}$. The data is correlated with a nuclear magnetic resonance study of enzyme-substrate interaction, and sequence comparison with two other reductases reveals fourteen positions of sequence identity. The sequence given is based upon mass spectrometric evidence, and represents part of a study involving the first major use of mass spectrometry in sequencing a protein of unknown structure in the absence of a concurrent classical strategy.