Abstract
Six mutations determining resistance to amethopterin were examined for their effects on the active transport of the drug. In strains bearing each of the mutations and exhibiting resistance levels varying from 10- to 100-fold, transport at limiting concentrations of H(3)-amethopterin was reduced from 2.5 to 10 times the rate characteristic of the wild type. Kinetic analysis of transport showed an increase in the value for K(m) of the system in all of the mutants. Values for the wild-type system were 0.9 x 10(-6)m and for the mutants varied between 2.5 x 10(-6)m and 9.0 x 10(-6)m. Values for V(max) were approximately the same for each system. The mutant transport systems also exhibited a shift in pH optimum from near 6.0 (wild-type) to below 5.0. The results were interpreted as an alteration in the binding properties of the permease in the mutant strains.