3. Which of the following is necessary for transport of m-RNA from nucleus
(1) Alternative Splicing
(2) 5′-Capping
(3) RNA editing
(4) Secondary structure

Necessary Factor for mRNA Transport from Nucleus in Eukaryotes

Correct Answer:

(2) 5′-Capping

Detailed Explanation:

The transport of mature mRNA from the nucleus to the cytoplasm is a tightly regulated process involving multiple steps and factors. Among these, 5′-capping of the mRNA is essential for efficient nuclear export.

• 5′-Capping occurs early during transcription, where a 7-methylguanosine cap is added to the nascent pre-mRNA. This cap is recognized by the cap-binding complex (CBC), which plays a critical role in mRNA processing and export.

• The cap and associated proteins help assemble an export-competent messenger ribonucleoprotein (mRNP) complex, which is then targeted to the nuclear pore complex (NPC) for export.

• The export receptors (e.g., NXF1/TAP) bind to the mRNP and mediate its translocation through the NPC into the cytoplasm.

• Proper capping ensures that the mRNA is recognized as mature and ready for export. Without the 5′ cap, mRNA export is inefficient or blocked, leading to nuclear retention and degradation.

Why Other Options Are Incorrect:

1. Alternative Splicing
   While splicing is important for mRNA maturation and can influence export, it is not strictly necessary for all mRNAs to be exported. Some intronless mRNAs are exported efficiently, and splicing alone does not guarantee export.

2. RNA Editing
   RNA editing modifies specific nucleotides post-transcriptionally but is not a general requirement for mRNA export.

3. Secondary Structure
   RNA secondary structure can influence stability and translation but does not directly mediate nuclear export.

Supporting Evidence from Literature:

• Carmody and Wente (2009) describe mRNA nuclear export as dependent on proper mRNP assembly, including the presence of the 5′ cap and associated proteins that mark the transcript for export.

• Structural studies highlight the role of the cap-binding complex and export receptors in targeting mRNPs to the nuclear pore complex for translocation.

• The TREX complex, which couples transcription and export, interacts with capped mRNAs to facilitate efficient nuclear export.

Summary Table

Conclusion

5′-capping of mRNA is necessary for its efficient transport from the nucleus to the cytoplasm. The cap structure and its binding proteins ensure that the mRNA is properly processed and recognized by the nuclear export machinery, facilitating its passage through the nuclear pore complex.