Scavenger decapping activity facilitates 5′ to 3′ mRNA decay

mRNA degradation occurs through distinct pathways, one primarily from the 5' end of the mRNA and the second from the 3' end. Decay from the 3' end generates the m7 GpppN cap dinucleotide, which is subsequently hydrolyzed to m(7)Gp and ppN in Saccharomyces cerevisiae by a scavenger decapping activity termed Dcs1p. Although Dcs1p functions in the last step of mRNA turnover, we demonstrate that its activity modulates earlier steps of mRNA decay. Disruption of the DCS1 gene manifests a threefold increase of the TIF51A mRNA half-life. Interestingly, the hydrolytic activity of Dcs1p was essential for the altered mRNA turnover, as Dcs1p, but not a catalytically inactive Dcs1p mutant, complemented the increased mRNA stability. Mechanistic analysis revealed that 5' to 3' exoribonucleolytic activity was impeded in the des1 Delta strain, resulting in the accumulation of uncapped mRNA. These data define a new role for the Des1p scavenger decapping enzyme and demonstrate a novel mechanism whereby the final step in the 3' mRNA decay pathway can influence 5' to 3' exoribonucleolytic activity.