FIG. 17.

A major route of mRNA decay in yeast. Specific elements in the body of the mRNA are proposed to dictate (possibly via interactions with Pab1p) the rate of deadenylation by a PAN-type enzyme (complex). Once a shortened tail has been generated, the major pathway involves the triggering of decapping by Dcp1p (or Dcp2p), which is then followed by 5′→3′ exonucleolytic decay (catalyzed by Xrn1p). The Mrt proteins may modulate decapping. Alternatively, exonucleolytic degradation from the 3′ end has been observed. It is now suspected that the multienzyme “exosome,” or something like it, is involved in this process.