Question

At times a ribosome may get stuck with an mRNA that has been damaged so that there is no stop codon. What happens to this ribosome to fix this problem?

Answer 1

During the process of translation, there might be a number of errors in the mRNAs that might halt the ribosome attached to it. These errors maybe absence of a stop codon from premature 3' adenylation or obscure polyadenylation signals. NOw absence of the stop codon rises to a serious issue in the cell as after the translation or spilicing procedure, the ribosome is unable to eject the mRNA and free itself. To fix this problem, there are two ways:

Nonstop mediated decay(NMD)- This mechanism frees the ribosome plus marks the defected mRNA for degradation by the nucleases in the cell. The signals in the non stop mRNA with no stop codon is destroyed by the NMD pathway. The NMD proteins bind to the region where the ribosome is stuck with the mRNA and releases the ribosome, thereby still bound to the mRNA and targeting the mRNA for protein degradation pathways

Trans-translation- This is mediated by transfer-messenger RNA (tmRNA) that binds to the free A site of the ribosome. This allows the ribosome to shift it's translation mechanism from the defective mRNA to the tmRNA until the ribosome reaches the stop codon of the tmRNA. In this way, the resultant truncated protein of the tmRNA is degraded along with the defective mRNA that was earlier bound to the ribosome