Transcription in Eukaryotes and Post-translational Modification Flashcards
What regions are required for transcription initiation in eukaryotes?
Transcription initiation requires promoter and upstream regulatory regions.
What are cis-acting elements and why are promoters called cis-acting elements?
Cis-acting elements are specific sequences on the DNA template that regulate the transcription of one or more genes. Promoters are called cis-acting elements because they are always on the same molecule of DNA as the genes being transcribed.
What is the role of cis-acting elements in transcription initiation?
Cis-acting elements serve as binding sites for proteins called transcription factors.
What is the TATA box and where is it located in eukaryotic promoters?
The TATA box is an element with the consensus sequence TATAAAA that begins about 25–30 base pairs upstream of the start of transcription in most eukaryotic promoters.
Further notes:
Transcription factors bind to cis-acting elements, and the TATA box is one such element where they can bind to initiate transcription.
The TATA box is recognized by ____(a)____, which helps recruit other transcription factors and RNA polymerase ____(b)____ to the promoter region, facilitating the initiation of transcription.
(a) TATA-binding protein (TBP)
(b) II
α-amanitin (from Amanita phalloides, a highly poisonous species of mushroom) is a specific inhibitor of RNA-polymerase. This toxin can cause fatal liver damage within 48 hours.
Comment of the sensitivity of various eukaryotic RNA polymerases to α-amanitin.
(1) RNA polymerase I is insensitive to α-amanitin.
(2) RNA polymerase II is highly sensitive to α-amanitin, with inhibition occuring at very low concentrations (around 1µg/ml). This makes it a primary target of the toxin.
(3) RNA polymerase III is moderately sensitive to α-amanitin, requiring higher concentrations (around 10 µg/ml) for inhibition compared to RNA polymerase II.
How does RNA polymerase bind to the promoter in eukaryotic systems compared to prokaryotic systems?
Unlike in prokaryotes, in eukaryotic systems, RNA polymerase does not bind the promoter directly and requires various protein factors (transcription factors) to help locate promoters and initiate transcription.
What role does TFIIH play in the phosphorylation of RNA polymerase II?
TFIIH has protein kinase activity that phosphorylates the C-terminal domain (CTD) of RNA polymerase II.
Further notes:
The CTD consists of a repetitive sequence of amino acids with the consensus sequence of YSPTSPS. The CTD plays a crucial role in the regulation of transcription and RNA processing.
What happens to RNA polymerase II after it is phosphorylated by TFIIH?
Only the phosphorylated RNA polymerase II can move downstream, starting the elongation phase.
What is one difference between the elongation process of DNA transcription in Eukaryotes and Prokaryotes?
Unlike in prokaryotes, in eukaryotic systems, transcription and translation do not take place simultaneously since they are separated by the nuclear membrane.
What is the termination sequence in eukaryotic transcription?
The termination sequence is AATAAA followed by GT repeats.
Primary transcripts of mRNA are called ________.
heteronuclear RNA (hnRNA)
What are four modifications that may happen to hnRNA?
(1) capping at the 5’ end
(2) tailing at the 3’ end
(3) mRNA splicing
(4) RNA editing
Explain capping at the 5’ end, a crucial step in post-translational modification.
It involves the addition of a modified guanine nucleotide, known as a 7-methylguanosine cap, to the 5’ end of the nascent RNA transcript. This cap:
(i) protects the mRNA from degradation
(ii) aids in ribosome binding during translation, and
(iii) is involved in the export of mRNA from the nucleus to the cytoplasm.
Explain poly-A tailing at the 3’ end, a crucial step in post-translational modification, and state its importance.
This process involves the addition of a long chain of adenine nucleotides, known as the poly-A tail, to the 3’ end of the RNA transcript. This serves three important functions:
(1) Stability: the poly-A tail enhances the stability of the mRNA by protecting it from degradation by exonucleases.
(2) Export: it facilitates the export of the mRNA from the nucleus to the cytoplasm.
(3) Translation: The poly-A tail aids in the initiation of translation by interacting with proteins that bind to the 5’ cap and the poly-A tail, forming a circular structure that promotes ribosome binding.
