Date of Award


Degree Type


Degree Name

Doctor of Philosophy


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 do wild-type cells to support growth at a similar level. Two classes of mutants were distinguishable by their revertability for growth on folinic acid and ability to take-up folic acid. Revertants derived from one class were similar to wild-type cells in their ability to grow in medium containing low levels of folinic acid and in their sensitivity to methotrexate. In contrast, revertants from a second class were able to grow in medium containing low or no folinic acid, but retained their methotrexate resistance. Somatic cell hybrids formed between these two classes of mutants were non-complementing. These observations suggested that some but not all components may be shared between the transport systems mediating methotrexate and folic acid uptake.;The second class of methotrexate-resistant Chinese hamster ovary cells have been complemented to methotrexate sensitivity by transfection with DNA isolated from either wild-type Chinese hamster ovary or human G2 cells. Primary and secondary transfectants regained the ability to take up methotrexate in a manner similar to wild-type cells. To assist in cloning the sequences responsible for this complementation, transfections have also been carried out with DNA from a wild-type cosmid library. Transfectants have been isolated which have regained methotrexate sensitivity, the ability to take-up methotrexate and were found to contain a limited number of transfected cosmid sequences. Three cosmid clones have been isolated from a primary methotrexate sensitive transfectant cosmid library which after being transfected into the mutant, rescued the methotrexate resistant phenotype of the mutant at a high frequency. Restriction endonuclease analysis of the cosmid clones determined that they overlapped extensively and shared, two regions of 6.6 kB and 20.9 kB DNA. These observations suggested that a gene involved in Mtx uptake is contained within these regions. This is the first report of the molecular cloning of a gene specific to Mtx uptake.



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