POLYSOME-ASSOCIATED MESSENGER-RNAS ARE SUBSTRATES FOR THE NONSENSE-MEDIATED MESSENGER-RNA DECAY PATHWAY IN SACCHAROMYCES-CEREVISIAE

In eukaryotic cells, premature termination of translation at nonsense codons has been implicated as the cause of a variety of posttranscript ional events, including rapid mRNA decay in the cytoplasm or the nucle us, altered splice site selection, and exon skipping. In the yeast Sac charomyces cerevisiae, nonsense codons promote accelerated mRNA decay, and we sought to determine the cellular location in which this degrad ation occurs. in this report, we demonstrate that six different mRNAs, including nonsense-containing transcripts of the LEU2, HIS4, PGK1, an d CYH2 genes, and two wild-type mRNAs (the MAT alpha 1 and CYH2 mRNAs) , were stabilized when the translation elongation inhibitor cyclohexim ide was added to cellular growth media. Subsequent removal of cycrohex imide resulted in resumption of translation and degradation of wild-ty pe and nonsense-containing mRNAs. A significant fraction of the CYH2 p re mRNA that accumulated in the presence of cycloheximide was associat ed with polysomes, but disappeared from that fraction when decay resum ed in the absence of the drug. Moreover, the abundance of the spliced and unspliced forms of the untranslated U3 snRNA was shown to be unaff ected in strains harboring mutations that stabilize nonsense-containin g mRNAs. Taken together, these observations indicate that nonsense-con taining mRNAs in yeast are degraded within the polysome compartment of the cell.