The isolation of a suppressible nonsense mutant in mammalian cells.

An HGPRT- cell line derived from mouse L cells has been shown to have the following properties: it is CRM+; the defective HGPRT molecules are altered in the carboxyterminal peptide; the mutant cells regain HGPRT activity when ochre-suppressor tRNA is microinjected into them, but not when amber-suppressor or wild-type tRNAs are injected. We conclude from these properties that this mutant cell line contains an ochre nonsense mutation (UAA) in the structural gene for HGPRT.