THE RELATIONSHIP BETWEEN EUKARYOTIC TRANSLATION AND MESSENGER-RNA STABILITY - A SHORT UPSTREAM OPEN READING FRAME STRONGLY INHIBITS TRANSLATIONAL INITIATION AND GREATLY ACCELERATES MESSENGER-RNA DEGRADATION INTHE YEAST SACCHAROMYCES-CEREVISIAE

A new strategy was developed to study the relationship between the tra nslation and degradation of a specific mRNA in the yeast Saccharomyces cerevisiae. A series of 5'-untranslated regions (UTR) was combined wi th the cat gene from the bacterial transposon Tn9, allowing us to test the influence of upstream open reading frames (uORFs) on translation and mRNA stability. The 5'-UTR sequences were designed so that the min imum possible sequence alteration, a single nucleotide substitution, c ould be used to create a 7-codon ORF upstream of the cat gene. The uOR F was translated efficiently, but at the same time inhibited translati on of the cat ORF and destabilized the cat mRNA. Investigations of var ious derivatives of the 5'-UTR indicated that cat translation was prim arily attributable to leaky scanning of ribosomes past the uORF rather than to reinitiation. Therefore, these data directly demonstrate dest abilization of a specific mRNA linked to changes in translational init iation on the same transcript. In contrast to the previously proposed nonsense-mediated mRNA decay pathway, destabilization was not triggere d by premature translational termination in the main ORF and was not d iscernibly dependent upon a reinitiation-driven mechanism. This sugges ts the existence of an as yet not described pathway of translation-lin ked mRNA degradation.