Summary: The type IIIb dihydrofolate reductase, a novel plasmid-encoded enzyme recently identified in , has been shown to have some similar biochemical properties to the type IIIa dihydrofolate reductase which was first identified in New Zealand in 1979. However, the type IIIb enzyme has a for trimethoprim of 0.4 μM, and a pI of 5.35 (as compared to 19 nM and 6.1 for the type IIIa); both these results suggest that it is a different enzyme from the prototype type IIIa. The type IIIb dihydrofolate reductase was purified by methotrexate agarose affinity chromatography, yielding a pure protein as determined by HPLC. Automatic amino acid analysis of the purified enzyme showed it to be distinct from all other known plasmid-encoded dihydrofolate reductases and quite different from the type IIIa enzyme. The purified enzyme was examined by SDS-PAGE, which revealed that the type IIIb dihydrofolate reductase was a monomeric protein of 17200.


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