Linkage of markers controlling consecutive biochemical steps in CHO cells as demonstrated by chromosome transfer

Using the technology of metaphase chromosome transfer, evidence has been obtained in CHO cells that genes controlling enzymes in a common pathway in folate metabolism are closely linked. Mtx^RI and Mtx^RIII are co-dominant mutations which affect the structure and level of dihydrofolate reductase. Gat^? is a glycine-, adenosine- and thymidine-requiring auxotrophic mutant with a lesion in folylpolyglutamate synthetase, an enzyme responsible for addition of glutamates to folate residues. GlyB^? is an auxotrophic glycine-requiring mutant whose phenotype may be reversed by folinic acid. Using purified metaphase chromosomes, the Mtx^R genes were co-transferred into recipient cells with the auxotrophic markers, as demonstrated by the isolation of transferents when two of the phenotypes, either Mtx and Gat, or Mtx and GlyB, were selected at the same time. When recipient cells were selected for Gat⁺ or GlyB⁺ alone, the transferents carried the Mtx^R markers. The GlyA mutation, another glycine-requiring auxotrophic change, is not co-transferred with methotrexate resistance.Because of previous evidence that only a small fragment is involved in chromosomal transfer experiments, these results seem to provide the first indication that some genes which control enzymes on a common metabolic pathway in eucaryotes are closely linked or are at least syntenic.