Transcription and RNA Processing

Overview of Transcription and RNA Processing

Transcription is the first step in gene expression, converting DNA information into RNA. In both prokaryotes and eukaryotes, this process is tightly regulated by promoter elements, transcription factors, and termination signals. After synthesis, the primary RNA transcript undergoes several processing steps—capping, splicing, and polyadenylation—before becoming a functional messenger RNA (mRNA) or a regulatory RNA.

Prokaryotic Transcription Initiation

Promoter Architecture

In bacteria, the sigma (σ) factor directs RNA polymerase to the promoter. Two conserved regions are critical:

• -35 element: consensus TTGACA
• -10 element (Pribnow box): consensus TATAAT

The sigma factor primarily recognizes the -35 region, but the -10 region is essential for DNA melting and open complex formation. Mutations that disrupt these consensus sequences reduce transcription efficiency.

Example Question

Which DNA sequence element is primarily recognized by the sigma factor to initiate transcription in prokaryotes?

• The -35 region with consensus TTGACA (Correct)
• The -10 region (Pribnow box) with consensus TATAAT
• The TATA box upstream of the transcription start site
• The CAAT box within the promoter

Impact of Promoter Mutations

Changing the -10 consensus from TATAAT to TATAGT weakens sigma factor binding, leading to reduced transcription efficiency. The -35 region alone cannot compensate for a defective -10 element because the open complex cannot form properly.

Prokaryotic Transcription Termination

Rho‑Independent (Intrinsic) Terminators

Intrinsic terminators consist of a GC‑rich hairpin followed by a poly‑U tract. The hairpin destabilizes the RNA‑DNA hybrid, and the weak rU‑dA base pairs cause the RNA polymerase to dissociate.

Rho‑Dependent Terminators

When a terminator is absent, the helicase protein Rho can bind a rut site downstream of the gene, translocate along the nascent RNA, and terminate transcription.

Example Questions

A bacterial gene terminates transcription via a rho‑independent mechanism. Which structural feature of the nascent RNA is essential for this termination?

• A GC‑rich hairpin loop followed by a poly‑U tract (Correct)
• A stretch of adenine residues forming a poly‑A tail
• A terminator sequence encoding a stop codon
• A rut site that binds the Rho helicase protein

A researcher observes that a bacterial gene lacking a functional terminator still produces a short RNA transcript. Which mechanism most likely accounts for this observation?

• Rho‑dependent termination occurring downstream of the gene (Correct)
• RNA polymerase backtracking and releasing the transcript
• Alternative sigma factor competition reducing transcription length
• Premature transcriptional pausing due to DNA secondary structures

Eukaryotic Transcription Initiation

Core Promoter Elements

Key motifs include the TATA box, the initiator (Inr), and downstream promoter elements. The first factor to bind the TATA box is the TATA‑binding protein (TBP), a subunit of the multi‑protein complex TFIID. TBP recruitment paves the way for other general transcription factors (GTFs) and RNA polymerase II.

Example Question

In eukaryotic transcription initiation, which factor first binds to the TATA box to recruit the rest of the preinitiation complex?

• TATA‑binding protein (TBP) as part of TFIID (Correct)
• RNA polymerase II holoenzyme without any transcription factors
• TFIIB binding to the initiator element (Inr)
• TFIIA directly recognizing the TATA box

RNA Processing in Eukaryotes

5' Capping and Polyadenylation

Immediately after initiation, the 5' end of the nascent transcript receives a 7‑methylguanosine cap, protecting it from exonucleases and facilitating ribosome binding. At the 3' end, the polyadenylation signal AAUAAA directs cleavage of the pre‑mRNA, after which a poly‑A tail is added. Importantly, transcription does not stop at the poly‑A signal; RNA polymerase II continues downstream until termination factors and the cleavage complex destabilize the polymerase.

Splicing and the Spliceosome

Introns are removed by the spliceosome, a dynamic assembly of small nuclear ribonucleoproteins (snRNPs). The U2 snRNP recognizes the branch point adenosine, while U1 binds the 5' splice site, U5 aligns the exons, and U6 catalyzes the second transesterification reaction.

Example Questions

During RNA splicing, which component of the spliceosome recognizes the branch point adenosine?

• U2 snRNP binds the branch point adenosine (Correct)
• U1 snRNP binds the 5' splice site
• U6 snRNP catalyzes the second transesterification
• U5 snRNP aligns the exons for ligation

Which of the following best explains why the polyadenylation signal AAUAAA does not cause immediate termination of RNA polymerase II transcription?

• Polymerase continues transcribing downstream until it is destabilized (Correct)
• The signal directly recruits a termination factor that halts transcription instantly
• The poly‑A tail is added before transcription termination, preventing polymerase release
• RNA polymerase II lacks a termination mechanism and relies on RNA cleavage only

RNA Interference (RNAi)

miRNA vs. siRNA

Both microRNAs (miRNAs) and small interfering RNAs (siRNAs) are ~21‑23 nucleotides long and associate with the RNA‑induced silencing complex (RISC). However, their mechanisms differ:

• miRNA: typically imperfectly base‑pairs with target mRNAs, leading to translational repression.
• siRNA: perfectly complementary to its target, guiding RISC to cleave the mRNA.

Example Question

Which statement correctly distinguishes the roles of miRNA and siRNA in RNA interference?

• miRNA typically represses translation, while siRNA directs target mRNA cleavage (Correct)
• Both miRNA and siRNA function identically by recruiting Dicer to cleave mRNA
• miRNA is processed by Dicer, whereas siRNA bypasses Dicer and directly loads into RISC
• miRNA induces mRNA degradation, while siRNA enhances translation

Key Take‑aways

• Prokaryotic promoters contain -35 and -10 elements; sigma factor binds primarily to the -35 region.
• Intrinsic terminators rely on a GC‑rich hairpin followed by a poly‑U tract; rho‑dependent termination uses the Rho helicase.
• In eukaryotes, TBP (part of TFIID) is the first factor to recognize the TATA box.
• RNA processing steps—capping, splicing, and polyadenylation—are essential for mature mRNA stability and export.
• U2 snRNP identifies the branch point; the spliceosome orchestrates precise intron removal.
• miRNA generally represses translation, whereas siRNA induces cleavage of perfectly matched targets.

Further Reading and Resources

To deepen your understanding, explore the following reputable sources:

• Molecular Biology of the Gene (NCBI Bookshelf)
• RNA Polymerase II Transcription Cycle (Nature Reviews Molecular Cell Biology)
• Spliceosome Mechanics (ScienceDirect)
• RNA Interference Tutorial (EMBL‑EBI)