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Genetically Alterable Transport of Amethopterin in Diplococcus pneumoniae II. Impairment of the System Associated with Various Mutant Genotypes

Division of Experimental Chemotherapy, Sloan-Kettering Institute for Cancer Research, Cornell University Medical College, New York
Sloan-Kettering Division, Graduate School of Medical Sciences, Cornell University Medical College, New York

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 H³-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^–6M and for the mutants varied between 2.5 x 10^–6M and 9.0 x 10^–6M. 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.