Question

In eukaryotic cells, mRNAs have been found to have a circular arrangement in which proteins hold the poly-A tail near the 5’ cap. How might this increase translation efficiency?

Short answer

If the eukaryotic mRNA were circular, then the poly-A tail present at 3’end of RNA would be near the 5’cap. This would enhance the rebinding of ribosomal subunits, which dissociates on termination because the subunits are close to the 5’cap.

Thus, polypeptide synthesis is reinitiated, and the new polypeptide is synthesized. As a result, translation efficiency increases.

Step-by-step solution

Structure of mature RNA transcript

The mRNA molecule synthesized from DNA through transcription is called pre-mRNA. The pre-mRNA undergoes RNA processing before they are released from the nucleus into the cytoplasm for translation.

RNA processing includes:

• Adding 5’ cap to the 5’ end of pre-mRNA.

• Adding poly-A tail to the 3’ end of pre-mRNA.

• Removing introns (non-coding regions) from pre-mRNA through splicing.

As a result, a mature mRNA transcript is produced that bears a 5’ cap and poly-A tail.

Role of ribosomes in translation

The translation is mediated through ribosomes. To initiate translation, the small unit of ribosome bounded to the initiator tRNA that carries the first amino acid of the polypeptide binds to the 5’cap of the mRNA molecule.

It recognizes the initiation codon on the mRNA molecule. This is followed by the attachment of the large subunit of the ribosome. The ribosome then moves along the mRNA molecule and subsequently synthesizes the polypeptide chain until it reaches the stop codon, where the translation is terminated.

Effect on circular mRNA molecule on translation efficiency

Once the translation is terminated, the subunits of the ribosome dissociate. However,if the RNA molecule is circular, the subunits are close to the 5’cap, where small subunits bind to initiate translation.

As a result, the subunits rebind and initiate the synthesis of a new polypeptide chain, thus, increasing the translation efficiency. Thus, circular eukaryotic mRNA can increase the efficiency of translation.