Fig. 7. Model of No-Go decay pathway involving Trl1 kinase and 5’–3’ exoribonucleases.

Top of figure, the third ribosome is represented as competent for NGD endonuclease activation. We propose that the two first stalled ribosomes are not properly conformed to trigger the endonucleolytic process. NGD endonuclease cleavage (lightning flash) occurs 8 nts upstream of the first P-site residue, within the mRNA exit tunnel of the ribosome. Upstream ribosomes covering the resulting 5′-NGD fragments can advance and stall on the new 3′-end with 1 nt in the ribosomal A-site. Colliding ribosomes on this new RNA fragment can induce a novel NGD endonuclease activation. On 3′-NGD RNAs, like B4 RNAs, the NGD-competent ribosome dissociates and facilitates access of Trl1 RNA kinase to the 5′-hydroxylated 3′-NGD RNA, but we cannot exclude that the leading ribosome dissociates and upstream ribosomes run to form a new disome with 5′-protruding RNA. Once the RNA is 5′-phosphorylated, the processive 5′–3′ exonucleolytic activity of Xrn1 can degrade and produce B1 RNA. Alternatively, upon Xrn1 inactivation, 5′–3′ exonucleolytic digestion of this RNA by Dxo1 can occur and produce trimmed RNAs, such as B3 and B2 RNAs. Middle of figure, upstream of the 3rd ribosome, ribosomes are also competent for NGD endonuclease activation. Here, the endonucleolytic cleavage occurs in the 4th ribosome and B5 RNAs can derive from such RNAs after phosphorylation and 5′–3′ digestion. Bottom of figure, alternative pathways are proposed: B1 and B5 RNA production could be initiated by decapping or via Hel2-independent uncharacterized endo/exonucleolytic attacks.