A synthetic A tail rescues yeast nuclear accumulation of a ribozyme-terminated transcript

To investigate the role of 3' end formation in yeast mRNA export, we replaced the mRNA cleavage and polyadenylation signal with a self-cleaving hammerhead ribozyme element. The resulting RNA is unadenylated and accumulates near its site of synthesis. Nonetheless, a significant fraction of this RNA reaches the cytoplasm. Nuclear accumulation was relieved by insertion of a stretch of DNA-encoded adenosine residues immediately upstream of the ribozyme element (a synthetic A tail). This indicates that a 3' stretch of adenosines can promote export, independently of cleavage and polyadenylation. We further show that a synthetic A tail-containing RNA is unaffected in 3' end formation mutant strains, in which a normally cleaved and polyadenylated RNA accumulates within nuclei. Our results support a model in which a polyA tail contributes to efficient mRNA progression away from the gene, most likely through the action of the yeast polyA-tail binding protein Pab1p.