Cloning of the type VII trimethoprim-resistant dihydrofolate reductase gene and identification of a specific DNA probe.

A 1.3 kb HindIII fragment encoding the type VII trimethoprim-resistant dihydrofolate reductase gene was cloned into pBR322. Unidirectional deletion of this cloned fragment with exonuclease III identified the start of the dihydrofolate reductase gene. An internal 300bp EcoRV fragment was identified which could be used as a specific non-radioactive DNA probe to distinguish bacteria carrying the type VII gene from those carrying genes encoding other known dihydrofolate reductase types.