Question

(Molecular Biology) You are very intrigued by the circular configuration of eukaryotic mRNAs during translation, and spend much of your free time using an electron microscope to visualize them in different cells. You come across a mutant cell line in which many mRNAs are not circular, but linear. Speculate as to what kind of mutation might be underlying this behavior. What consequences might it have for translation in the mutant cells?

Answer 1

The circularization of the mRNA during translation is aided by the 5' cap and the polyA tail that are present on mature mRNAs.

The circularization is mediated by the eif4E and Pab1P which are 5'cap and polyA tail binding proteins respectively. This happens through the interactions of these proteins with different parts of the translation factor eif4G.  

the circularization of the mRNAs may be prevented due to mutations in either the capping or tailing mechanisms of mRNAs,

or due to mutant cap-binding and polyA tail binding proteins. Mutations in the sequence of the mRNAs preventing their capping or tailing may also be responsible for the linear form of mRNA seen in the mutant cells.

The consequence of these mutations may include,

• no translations initiation
• circularization may function as a proofreading mechanism in the translation of appropriately capped and tailed mRNAs, disruption in which may lead to translation of incorrectly processed mRNAs.
• recycling of factors occurs more efficiently when circularized which may be disturbed due to mutations
• translation regulation may be disrupted due to mutations causing linear mRNAs.
• the reinitiation of translation that occurs due to circularization will also be disrupted and the copy no. of the desired protein product may reduce.