Genetics Molecular Biology>Transcription Rna Processing Practice Test

Eukaryotic pre-mRNAs transcribed by RNA polymerase II undergo several processing steps before export and translation. A research team studied the earliest modification on nascent transcripts. They observed that, within seconds of emergence from the polymerase, the 5' end of the RNA became resistant to 5' to 3' exonucleases. Co-immunoprecipitation revealed recruitment of a nuclear cap-binding complex to the 5' terminus, which was later replaced by the cytoplasmic translation factor eIF4E. Treatment of extracts with a decapping enzyme restored 5' end vulnerability, whereas treatment with a deadenylase had no effect on 5' stability. The presence or absence of this protective 5' modification did not alter splice-site choice, and introns were removed efficiently in nuclear extracts regardless of decapping status. The investigators concluded that this modification at the extreme 5' end promotes nuclear export and translation initiation while shielding the transcript from 5' exonucleases, but it does not change exon composition.

A single human gene was found to produce two protein isoforms with distinct C-termini in different tissues. In neuronal cells, a middle exon containing a premature stop codon was skipped, yielding a longer protein; in hepatic cells, the exon was included, producing a shorter isoform. Knockdown of serine-arginine (SR) proteins shifted exon usage and changed the isoform ratio without affecting transcription rates. Both tissues exhibited robust 5' end protection against 5' exonucleases, and the 3' ends were equally stabilized by poly(A) binding protein, indicating that capping and polyadenylation were intact and comparable. Polysome profiling showed that both isoforms were translated efficiently once produced. The researchers concluded that tissue-specific regulatory factors influenced the selection of splice sites on the same pre-mRNA to generate different mRNAs encoding distinct protein products.