The role of Upf proteins in modulating the translation read-through of nonsense-containing transcripts

The yeast UPF1, UPF2 and UPF3 genes encode transacting factors of the nonse nse-mediated mRNA decay pathway, In addition, the upf1 Delta strain demonst rates a nonsense suppression phenotype and Upf1p has been shown to interact with the release factors eRF1 and eRF3. In this report, we show that both upf2 Delta and upf3 Delta strains demonstrate a nonsense suppression phenot ype independent of their effect on mRNA turnover. We also demonstrate that Upf2p and Upf3p interact with eRF3, and that their ability to bind eRF3 cor relates,vith their ability to complement the non-sense suppression phenotyp e, Iri vitro experiments demonstrate that Upf2p, Upf3p and eRF1 compete,vit h each other for interacting with eRF3. Conversely, Upf1p binds to a differ ent region of eRF3 and Can form a complex with these factors, These results suggest a sequential surveillance complex assembly pathway, which occurs d uring the premature translation termination process. We propose that the ob served nonsense suppression phenotype in the upf Delta strains can be attri buted to a defect in the surveillance complex assembly.