Isolation and characterization of norleucine-resistant baby hamster kidney cells

Variants resistant to low and high levels of the methionine analogue norleucine were isolated in baby hamster kidney cells and in two clonal sublines, B1 and TG2. Clones resistant to high levels of norleucine were observed only after chemical mutagenesis, whereas clones capable of growing in low concentrations of norleucine occurred with equal frequency spontaneously and after mutagenesis. The variants were characterized with respect to uptake of ¹⁴C-norleucine and ¹⁴C-methionine. Five clones were found to be deficient in ¹⁴C-norleucine uptake, and of these, four showed reduced ¹⁴C-methionine uptake. The variants were tested also for increased activity of N₅-methyl-tetrahydrofolate: homocysteine methyltransferase, the enzyme which catalyses the terminal reaction in methionine biosynthesis. In four clones, higher levels of the methyltransferase were present than in the wild-type cells, suggesting overproduction or stabilization of this enzyme.