The invention relates to the nucleic acid sequence and amino acid sequence of dihydrofolate reductase (DHFR) from mycobacteria and to expression of recombinant DHFR protein. Utilizing the recombinant protein, novel therapies and diagnostic strategies can be developed and selective antimycobacterial compositions can be designed and utilized to treat mycobacterial infections in patients. This invention includes all or portions of novel recombinant nucleic acids encoding DHFR for mycobacteria such as M. avium, to novel recombinant DHFR peptides produced by such sequences, and to vaccines, diagnostic kits, cells and therapies utilizing these peptides and nucleic acid sequences. The present invention relates to methods for using the sequences of the present invention to develop drugs specific to M. avium and other mycobacteria, to identify and sequence corresponding sequences in species other than M. avium, as well as diagnostic and treatment methods incorporating the disclosed sequences and peptides.