20. In order to ensure that only fully processed mature mRNAs are allowed to be exported to cytosol, pre- mRNAs associated with snRNPs are retained in the nucleus. To demonstrate this, an experiment was performed where the gene coding a pre-mRNA, with a single intron was mutated either at 5’or 3′ splice sites or both the splice site. Given below are a few possible outcomes:
A. Pre-mRNA having mutation at both the splice sites will be retained in the nucleus because of the presence of bound snRNPs
B. Pre-mRNA having mutation at both the splice sites will be exported to cytosol.
C. Pre-mRNA mutated at either 3′ or 5′ splice sites will be retained in the nucleus because of the presence of bound snRNPs
D. Pre-mRNA mutated at either 3′ or S’ splice sites will be exported to cytosol because of the absence of bound snRNPs
Choose the correct combination of possible outcomes:
(1) B and C    (2) A and D
(3) Band D   (4) A and C

Nuclear Retention and Export of Pre-mRNAs with Splice Site Mutations: Experimental Outcomes Explained

In eukaryotic cells, only fully processed mature mRNAs are exported from the nucleus to the cytoplasm for translation. Pre-mRNAs that contain introns and are associated with spliceosomal small nuclear ribonucleoproteins (snRNPs) are typically retained in the nucleus until splicing is complete. This quality control mechanism prevents export of faulty or unprocessed transcripts.

The Experiment

A gene coding for a pre-mRNA with a single intron was mutated at either the 5′ splice site, the 3′ splice site, or both. The question is how these mutations influence nuclear retention or export of the pre-mRNA.

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Possible Outcomes and Their Interpretation

• A. Pre-mRNA with mutations at both splice sites will be retained in the nucleus because of bound snRNPs.
  This is incorrect because mutations at both splice sites generally prevent snRNP binding and spliceosome assembly, so the pre-mRNA is less likely to be retained by snRNP-mediated quality control.

• B. Pre-mRNA with mutations at both splice sites will be exported to the cytosol.
  This is correct. Without proper splice sites, snRNPs cannot bind effectively, and the transcript may escape nuclear retention and leak into the cytoplasm.

• C. Pre-mRNA mutated at either 3′ or 5′ splice site will be retained in the nucleus because of bound snRNPs.
  This is correct. Mutation at one splice site may still allow partial snRNP binding and spliceosome assembly, leading to nuclear retention.

• D. Pre-mRNA mutated at either 3′ or 5′ splice site will be exported to cytosol because of absence of bound snRNPs.
  This is incorrect. Partial splice site mutations often still recruit snRNPs, resulting in retention rather than export.

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Supporting Evidence from Literature

• Pre-mRNAs with suboptimal or mutated splice sites tend to be retained in the nucleus due to incomplete or stalled spliceosome assembly.

• Complete loss of splice sites prevents snRNP binding, allowing transcripts to escape nuclear retention and leak into the cytoplasm.

• Nuclear retention is linked to the presence of spliceosomal complexes; absence of these complexes reduces retention.

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Correct Combination of Outcomes

Based on the above, the correct combination is:

(1) B and C

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Summary

• Mutations at both splice sites prevent snRNP binding, allowing export of unprocessed pre-mRNA.

• Mutations at either 5′ or 3′ splice site allow partial snRNP binding, leading to nuclear retention.

• This mechanism ensures only properly spliced mRNAs are exported, maintaining RNA quality control.