Immunological characterization of hypoxanthine‐guanine phosphoribosyl transferase mutants of mouse L cells: Evidence for mutations at different loci in the HGPRT gene

A large collection (105) of mouse L cell mutants lacking hypoxanthine‐guanine phosphoribosyl transferase activity (HGPRT; E. C. 2.4.2.8) were analyzed for the presence of serologically cross reacting material (CRM). Antibody directed against highly purified mouse liver HGPRT was used for detecting CRM activity by two methods: (1) the standard precipitation‐inhibition assay; and (2) a radioimmune‐precipitation assay. The latter assay proved to have far greater sensitivity for the detection of altered forms of HGPRT. Approximately 40% of the HGPRT − cell lines contain CRM activity (i.e., were CRM + ). This indicates that a minimum of 40% of the HGPRT − clones arose as a result of mutations in the HGPRT structural gene. The CRM + cell lines were shown to contain different levels of CRM activity. Measurements of the heat sensitivity of CRM in the different HGPRT − cell lines showed a broad spectrum of CRM heat inactivation kinetics. These latter two observations provide strong evidence that the mutations giving rise to the HGPRT − CRM + phenotype occurred at different sites in the HGPRT structural gene.