Mutant Chinese hamster ovary cells with defective methotrexate uptake are distinguishable by reversion analysis

Chinese hamster ovary cells that are about 50x more resistant to the cytotoxic action of methotrexate than wild-type cells were deficient in the ability to take up methotrexate. In the absence of any exogeneous folates, these cells require 100-250x the level of folinic acid as wild-type cells to support growth at a similar level. Two classes of mutants were distinguishable by their revertability for growth on folinic acid. Revertants derived from one class were similar to wild-type cells in both their ability to grow in medium containing low levels of folinic acid and in their sensitivity to methotrexate. In contrast, partial revertants from a second class were able to grow in medium containing low or no folinic acid, but retained their methotrexate resistance. Furthermore, mutants of the first class were unable to take up folic acid while the second class of mutants accumulated folic acid to levels similar to that of wild-type cells. Somatic cell hybrids formed between these two classes of mutants were noncomplementing. These observations suggested that some, but not all, components may be shared between the transport systems mediating methotrexate and folic acid uptake.