Translation and a 42-nucleotide segment within the coding region of the mRNA encoded by the MAT alpha 1 gene are involved in promoting rapid mRNA decay in yeast.

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Abstract
In yeast, the mRNA encoded by the MAT.alpha.1 gene is unstable (t1/2 = 5 min) and the mRNAs encoded by the ACT1 gene (T1/2 = 30 min) and the PGK1 gene (t1/2 = 45 min) are stable. To understand the RNA structural features that dictate mRNA decay rates in yeast, we have constructed PGK1/MAT.alpha.1 and ACT1/MAT.alpha.1 gene fusions and analyzed the decay rates of the resultant chimeric transcripts. Fusion of a MAT.alpha.1 segment containing 73% of the coding region and the 3'' untranslated region to either of the stable genes is sufficient to cause rapid decay of the chimeric mRNAs (t1/2 = 6-7.5 min). Sequences required for this rapid decay are not found in the NAT.alpha.1 3'' untranslated region but are located within a 42-nucleotide segment of the coding region that has a high content (8 out of 14) of rare codons. Introduction of a translated stop codon upstream of this region stabilizes the hybrid mRNAs, indicating that the rapid decay promoted by these sequence is dependent on ribosomal translocation.