Mouse 3T6 cells that overproduce glutamine synthetase.

A mouse 3T6 subline that grows in glutamine-free medium has been cloned and exposed to a regimen of increasing concentrations of the glutamine synthetase inhibitor, methionine sulfoxime. Cells selected for resistance to 700 microM methionine sulfoxime show a 75-fold increase in glutamine synthetase activity relative to the original subclone. Immune precipitation of extracts prepared from cells pulse-labeled with L-[35S] methionine indicates that the increase in enzyme activity reflects an increase in biosynthesis of glutamine synthetase. Results obtained from in vitro translation followed by immune precipitation suggests that the methionine sulfoxime-resistant cells are highly enriched in mRNA encoding glutamine synthetase. The increase in enzyme activity is lost upon culture of the cells in nonselective medium--a finding consistent with the observation of double minute chromosomes in only the drug-resistant cells. These data strongly support the notion that methionine sulfoxime treatment has resulted in selection of cells that have amplified the gene encoding glutamine synthetase.