TRANSLATIONAL TERMINATION EFFICIENCY IN MAMMALS IS INFLUENCED BY THE BASE FOLLOWING THE STOP CODON

The base following stop codons in mammalian genes is strongly biased, suggesting that it might be important for the termination event. This proposal has been tested experimentally both in vivo by using the huma n type I iodothyronine deiodinase mRNA and the recoding event at the i nternal UGA codon and in vitro by measuring the ability of each of the 12 possible 4-base stop signals to direct the eukaryotic polypeptide release factor to release a model peptide, formylmethionine, from the ribosome, The internal UGA in the deiodinase mRNA is used as a codon f or incorporation of selenocysteine into the protein, Changing the base following this UGA codon affected the ratio of termination to selenoc ysteine incorporation in vivo at this codon: 1:3 (C or U) and 3:1 (A o r G). These UGAN sequences have the same order of efficiency of termin ation as was found with the in vitro termination assay (4th base: A ap proximate to G > > C approximate to U), The efficiency of in vitro ter mination varied in the same manner over a 70-fold range for the UAAN s eries and over an 8-fold range for the UGAN and UAGN series, There is a correlation between the strength of the signals and how frequently t hey occur at natural termination sites, Together these data suggest th at the base following the stop codon influences translational terminat ion efficiency as part of a larger termination signal in the expressio n of mammalian genes.