Relative argininosuccinate synthetase mRNA levels and gene copy number in canavanine-resistant lymphoblasts.

Mutants resistant to the arginine analogue, canavanine, have been isolated from two normal lymphoblast lines, MGL8B2 and MGL33. These mutants constitutively express up to 200-fold higher amounts of structurally normal argininosuccinate synthetase, the urea cycle enzyme that converts citrulline to argininosuccinate. Relative levels of argininosuccinate synthetase mRNA were compared among normal and canavanine-resistant lines using in vitro translation of poly(adenylic acid) RNA and blot hybridization of total cytoplasmic RNA to an argininosuccinate synthetase cDNA. Both of these approaches indicated that the canavanine-resistant lines contain increased steady-state levels of synthetase-specific mRNA relative to their sensitive parents and that these were roughly correlated with levels of enzyme activity. Blot hybridization of EcoRI-digested genomic DNA preparations revealed no detectable differences in argininosuccinate synthetase structural gene copy number between normal and canavanine-resistant lymphoblasts, demonstrating that the canavanine-resistant phenotype is not caused by gene amplification.