Transcription termination in eukaryotes | Eukaryotic transcription part 2

Summary

This video delves into the process of eukaryotic transcription termination, exploring the key events and molecular factors involved. It explains the critical steps of adding a poly-A tail and a 5' cap to the mRNA, highlighting the roles of specific proteins like CPSF, CSTF, and poly-A polymerase. The video also discusses the importance of these modifications in protecting the mRNA and ensuring its stability. Finally, it touches on the mechanisms for RNA polymerase dissociation and the subsequent splicing of introns, emphasizing how these processes contribute to proper gene expression and mRNA maturation in eukaryotic cells.

Takeaways

• 😀 Eukaryotic transcription termination involves multiple complex steps, requiring coordination of several proteins and factors.
• 😀 Unlike prokaryotic transcription, where termination is relatively simple, eukaryotic termination requires additional modifications like polyadenylation and capping.
• 😀 The start site of transcription is slightly downstream of the promoter, with the termination site defined further downstream.
• 😀 The polyadenylation signal triggers the addition of a poly-A tail at the 3' end of the mRNA, signaling the end of transcription.
• 😀 The transcription termination process in eukaryotes requires cleavage factors, such as CPSF and CSTF, to cleave the mRNA at the polyadenylation signal.
• 😀 Poly-A polymerase adds a stretch of adenine residues to the 3' end of the mRNA without a template, utilizing ATP as a source for adenine.
• 😀 The addition of the poly-A tail and the 5' cap is crucial for mRNA stability, lifespan, and efficient translation in eukaryotic cells.
• 😀 To prevent the formation of secondary RNA structures that could lead to miRNA formation, poly-A binding proteins (PBP) bind to the poly-A tail.
• 😀 RNA polymerase continues to synthesize mRNA even after the cleavage until a dissociation signal is triggered, which leads to the release of RNA polymerase from the DNA template.
• 😀 The exact mechanism of RNA polymerase dissociation is still debated, but it may involve a conformational change or the absence of a 5' cap in the nascent RNA.

Q & A

What is the primary purpose of transcription termination in eukaryotes?

Transcription termination ensures that RNA polymerase stops transcription at the correct location, allowing the RNA transcript to be properly processed and the polymerase to be recycled for future transcription rounds.

How does eukaryotic transcription termination differ from prokaryotic termination?

Prokaryotic termination often uses specific terminator sequences that form hairpin loops or rely on Rho helicase. In contrast, eukaryotic termination is more complex and depends on polyadenylation signals, cleavage factors, and post-transcriptional modifications like poly-A tail addition.

What role does the polyadenylation signal play during transcription termination?

The polyadenylation signal acts as a cue indicating that transcription is nearing its end. It triggers binding of CPSF and CSTF and initiates cleavage and polyadenylation of the RNA.

What are CPSF and CSTF, and what do they do?

CPSF (Cleavage and Polyadenylation Specificity Factor) and CSTF (Cleavage Stimulation Factor) are proteins that bind to the C-terminal tail of RNA polymerase II. Upon encountering the polyadenylation signal, they transfer to the RNA and help recruit cleavage factors for RNA processing.

What happens after CPSF and CSTF bind to the polyadenylation region?

They recruit additional cleavage factors, which act as endonucleases to cut the RNA downstream of the polyadenylation signal, releasing the primary RNA transcript.

What is the role of poly-A polymerase (PAP) after cleavage?

Poly-A polymerase adds a long stretch of adenine residues (poly-A tail) to the 3′ end of the cleaved RNA without needing a template, using ATP as the adenine source.

Why is the poly-A tail important?

The poly-A tail increases mRNA stability, protects it from degradation, and helps with nuclear export and translation efficiency.

What is the function of poly-A binding proteins (PABPs)?

PABPs bind to the poly-A tail to prevent unwanted secondary structures, such as double-stranded RNA formation, which could lead to harmful processes like microRNA formation.

How does RNA polymerase II dissociate from DNA after termination?

Two hypotheses exist: (1) a conformational change in RNA polymerase after transcript cleavage reduces its affinity for DNA, causing it to detach; (2) continued RNA synthesis lacking 5′ capping signals the polymerase to disengage.

What processing steps occur after transcription termination?

After termination, the pre-mRNA—now containing a 5′ cap and poly-A tail—undergoes splicing to remove introns and join exons before becoming a mature, functional mRNA.

Why must capping occur early, as soon as the 5′ end emerges?

Early capping protects the mRNA from degradation and is necessary for efficient processing, transport, and translation.

Why does RNA polymerase continue transcribing briefly after cleavage?

Polymerase continues transcribing because cleavage occurs on the RNA transcript, not on the polymerase itself. The polymerase requires an additional signal or conformational change to dissociate from the DNA.