CYTOPLASMIC MESSENGER-RNA FOR HUMAN TRIOSEPHOSPHATE ISOMERASE IS IMMUNE TO NONSENSE-MEDIATED DECAY DESPITE FORMING POLYSOMES

Nonsense codons between position 14 within the first exon and position 193 within the penultimate exon of the human gene for triosephosphate isomerase reduce mRNA abundance to 25% of normal. The reduction in ab undance is due to the decay of newly synthesized mRNA that copurifies with nuclei. TPI mRNA that copurifies with cytoplasm is immune to deca y. We show here that immunity is not due to the failure of nonsense-co ntaining mRNA to form polysomes. This finding indicates that cytoplasm ic mRNA, in contrast to nucleus-associated mRNA, may have lost one or more factors that are required for nonsense-mediated decay or gained o ne or more factors that confer immunity to nonsense-mediated decay.