5.18.2. Non-stop decay.
Non stop mRNA lacks stop codon i.e the mutation in DNA create a condition in which the stop codon of mRNA is converted into a sense codon and allow translation to continue. Translation of a mRNA which lacks a stop codon results in ribosomes traversing the poly(A) tail, displacing poly(A)-binding protein (PABP) and stalling at the 3' end of the mRNA. To release the ribosome form mRNA releasing factor binds with stop codon and later ribosome recycling factor dissociates the ribosome form mRNA.
In yeast and mammalian cells, Ski7 play a role in non stop decay. Ski7 is an adaptor protein that functions as a molecular mimic of tRNA, binds to the A site on the stalled ribosome to release the transcript, and then recruits the exosome. The exosome degrades the poly(A) tail and later the complete mRNA .
In another pathway described in Saccharomyces cerevisiae, in the absence of Ski7, the displacement of PABP by the translating ribosome renders the mRNA susceptible to decapping and 5' 3' decay by the 5' 3' exoribonuclease Xrn1.
5.18.3. An another mechanism for decay of mRNA
Several time because of the strong secondary RNA structure formation within the open reading frame (ORF) the ribosomes stall on the mRNA. That means the ribosome is not able to move on the mRNA thus called as No-go decay.
The Dom34 and Hbs1 proteins bind the transcript near the stalled ribosome and initiate an endonucleolytic cleavage event near the stall site. This releases the ribosome and generates two mRNA fragments, each with a free end exposed for exonucleolytic decay by the exosome and Xrn1, respectively.