FIGURE 1:

(A) A schematic depicting a typical yeast gene whose transcription profile is made up of multiple clusters (numbered from 5’ to 3’). Major mRNA isoforms within clusters are indicated by small gray triangles. A stability element (green rectangle) is present in major isoform 4 but absent in isoform 3 and shorter mRNA species.

(B) Major isoform 4 possesses a longer half-life than isoform 3 due to the presence of a stability element. In this model, isoform 3 has no stability element and is readily degraded by the exosome complex (yellow Pac-Man shape) from the 3’ to 5’ direction. The presence of either a polyU-poly(A) tail structure or a stem-loop near the 3’ terminus of isoform 4 blocks exosome-dependent degradation.