TK Locus in a Mutational Assay System

The majority of L5178Y cells, when plated in THMG-supplemented cloning medium, develop as THMG-resistant clones. One of these clones, 7X+/+- 7, vvas grown in suspension culture and served as the starting stock from which all subsequent mutant lines were ultimately derived. Table 6 confirms that this cell line is indeed TK competent. 

The TK-deficient cell line, TK —1.1 (see Table 6 for TK activity), was grown from the single BUdR-resistant colony recovered after plating 

A number of these presumed heterozygotes, as well as homozygotes, have been assayed for thymidine kinase activity. The results (Table 6) 

TABLE 7. Spontaneous Back-Mutation Rate of TA -/- Cells in Nonselective Medium  

Number of generations in nonselective medium Number of viable cells plated Number of TK+/ mutants Mutant frequency (mutations/locus) Mutation rate (mutations/locus/generation) 

---