Following statements were made with respect to transcription in eukaryotes: A. RNA polymerase III synthesises mRNAs in the nucleoplasm B. The target promoter for RNA polymerase III is usually represented by a bipartite sequence downstream of the transcription start site. C. The assembly factors TFIIIA and TFIIIC assist the binding of the positioning factor TFIIIB at the precise location. D. TFIIIB is the last factor that joins the initiation complex. E. Phosphorylated Ser residues in the C-terminal domain (CTD) of RNA polymerase II serve as binding sites for mRNA processing enzymes. Which one of the following options represents the correct combination of the statements?
B, C and E
Understanding Eukaryotic Transcription
Transcription is the process where a DNA sequence is copied into an RNA molecule. In eukaryotes, this process involves multiple RNA polymerases and complex regulatory mechanisms. Let's analyze each statement provided regarding transcription in eukaryotes.
We will examine each statement individually to determine its accuracy in the context of eukaryotic transcription.
Statement Analysis
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A. RNA polymerase III synthesises mRNAs in the nucleoplasm
This statement is incorrect. In eukaryotes, mRNA synthesis is primarily carried out by RNA polymerase II. RNA polymerase III is responsible for transcribing genes encoding transfer RNAs (tRNAs), 5S ribosomal RNA (5S rRNA), and some other small functional RNAs, not messenger RNAs (mRNAs).
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B. The target promoter for RNA polymerase III is usually represented by a bipartite sequence downstream of the transcription start site.
This statement is correct for many class II RNA polymerase III genes, such as those encoding tRNA and 5S rRNA. These genes often contain internal control regions (ICRs) within the transcribed region, downstream of the transcription start site. These internal sequences serve as promoter elements.
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C. The assembly factors TFIIIA and TFIIIC assist the binding of the positioning factor TFIIIB at the precise location.
This statement is correct. For RNA polymerase III transcription, specific transcription factors are required. TFIIIA (for 5S rRNA genes) and TFIIIC are assembly factors that bind to the internal promoter elements (like the bipartite sequence mentioned in B). Their binding facilitates the recruitment and stable binding of the positioning factor, TFIIIB, to a region upstream of the transcription start site. TFIIIB then serves as the docking site for RNA polymerase III.
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D. TFIIIB is the last factor that joins the initiation complex.
This statement is incorrect. While TFIIIB is crucial for positioning, it is the RNA polymerase III enzyme itself that joins the complex after TFIIIB has bound. TFIIIB binds to the DNA upstream of the transcription start site, guided by TFIIIA and TFIIIC (for internal promoters). RNA polymerase III then associates with the TFIIIB-DNA complex to form the complete pre-initiation complex before transcription begins. Therefore, RNA polymerase III joins after TFIIIB.
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E. Phosphorylated Ser residues in the C-terminal domain (CTD) of RNA polymerase II serve as binding sites for mRNA processing enzymes.
This statement is correct. RNA polymerase II, which transcribes pre-mRNA, has a large C-terminal domain (CTD) composed of multiple repeats of a heptapeptide sequence. Phosphorylation of serine residues within these repeats, particularly Ser2 and Ser5, is a dynamic process that occurs during transcription elongation. This phosphorylated CTD acts as a binding platform for various enzymes and factors involved in mRNA processing events such as capping, splicing, and polyadenylation, coupling transcription and processing.
Conclusion on Statements
Based on the analysis:
- Statement A is incorrect (RNA Pol II synthesizes mRNA).
- Statement B is correct (RNA Pol III internal bipartite promoters).
- Statement C is correct (TFIIIA and TFIIIC assist TFIIIB binding).
- Statement D is incorrect (RNA Pol III joins after TFIIIB).
- Statement E is correct (Phosphorylated CTD binds processing enzymes).
The correct combination of statements is B, C, and E.
Important Questions from RNA synthesis and processing
Which one of the following statements related to transcription and processing of mRNA is INCORRECT?
Following statements were made about the post-transcriptional processing of RNA in eukaryotes.
A. Soon after transcription initiation, RNA polymerase II pauses ~30 nucleotides downstream from the site of initiation until the Cap structure is added to the 5' end of the nascent pre- mRNA.
B. The 5' splice sites are functionally divergent whereas the 3' sites are functionally equivalent.
C. In addition to helping in recognition of the splice sites, the exon definition also functions as a splicing regulator by allowing pairing and linking of adjacent 5' and 3' splice sites.
D. The intron definition mechanism applies only to the larger introns (above 500 nucleotides length) and assists in achieving alternate splicing.
E. The splicing reactions carried out in vitro have revealed that the first and second transesterification reactions are reversible.
Which one of the following combination of statements is correct?
Which one of the following statements is NOT a correct feature of Escherichia coli RNA polymerase?
The following statements are related to transcription in bacteria/eukaryotes.
A. During concurrent promoter sequence recognition and melting, melting commences with base flipping where two bases are flipped out into pockets of the primary sigma factor
B. Binding of α-amanitin to RNA polymerase Il permits entry of nucleotides into RNA pol II active site and synthesis of RNA, but prevents translocation
C. RNA polymerase I can use upstream promoters with 3 consensus sequences, as well as internal promoters having a bipartite structure
D. FACT is associated with RNA polymerase during transcriptional elongation and helps displace histone octomers during transcription
Which of the following combinations of statements represents all correct statements?
Which one of the snRNAs given below base pairs with 5' splice site of pre mRNA?
