Abstract
Three spontaneous fol regulatory mutants contain dihydrofolate reductase molecules which differ in physical properties from enzymes of their parent strains. The enzymes were purified over 100-fold by affinity chromatography and were shown to differ in vitro in thermolability and in affinity for trimethoprim, a competitive inhibitor of the enzyme. These results indicate that some fol regulatory mutations occur in the structural gene for dihydrofolate reductase.