The initiation of transcription is a complex process involving promoter recognition, conversion of the initiation complex from closed to open form, abortive initiation events, and finally promoter escape. The following statements are made regarding these steps in transcription initiation: A. Promoter escape in bacteria is usually accompanied by the release of the sigma factor from the RNA polymerase holoenzyme complex. B. Abortive initiation events in prokaryotes result in the formation of short transcripts ~10 nucleotides in length while such events in eukaryotes result in formation of transcripts ~75 nucleotides in length. C. Promoter escape in eukaryotes is accompanied by the phosphorylation of the RNA polymerase large subunit on its C‐terminal domain (CTD). D. Promoter recognition in bacteria is governed by the sigma factor which binds to the ‐10 and ‐35 regions of the promoter followed by recruitment of the RNA Pol II core enzyme to form the holoenzyme. Which one of the following options represents the combination of all correct statements?
A and C only
Transcription Initiation Process
Transcription initiation is the crucial first step of gene expression, where RNA polymerase binds to a promoter sequence and begins synthesizing an RNA molecule. This complex process involves several stages, including promoter recognition, formation of the initiation complex, transition from closed to open complex, potential abortive initiation events, and finally, promoter escape to begin productive elongation.
Analyzing Transcription Initiation Statements
Let's evaluate each statement provided regarding the steps in transcription initiation in bacteria and eukaryotes.
Statement A: Promoter escape in bacteria is usually accompanied by the release of the sigma factor from the RNA polymerase holoenzyme complex.
In bacteria, the sigma factor is essential for recognizing the promoter and binding the RNA polymerase holoenzyme. Once the RNA polymerase successfully initiates transcription and moves a short distance away from the promoter (promoter escape), the sigma factor typically dissociates from the core enzyme. This dissociation allows the core enzyme to proceed with elongation more efficiently. This statement is correct.
Statement B: Abortive initiation events in prokaryotes result in the formation of short transcripts ~10 nucleotides in length while such events in eukaryotes result in formation of transcripts ~75 nucleotides in length.
Abortive initiation is common in both prokaryotes and eukaryotes. It involves the synthesis of short RNA fragments (typically less than 10-12 nucleotides) before productive elongation begins. If the RNA polymerase cannot successfully synthesize a longer, stable transcript and escape the promoter, it releases the short RNA and restarts the process. While abortive initiation occurs in both domains, the claim that eukaryotic abortive transcripts are around 75 nucleotides long is generally incorrect. Abortive transcripts in eukaryotes are also usually very short, often less than 10-15 nucleotides. Therefore, this statement is incorrect regarding the typical length of abortive transcripts in eukaryotes and the claimed difference.
Statement C: Promoter escape in eukaryotes is accompanied by the phosphorylation of the RNA polymerase large subunit on its C−terminal domain (CTD).
In eukaryotes, RNA polymerase II, which transcribes protein-coding genes, has a large subunit with a C-terminal domain (CTD) consisting of many repeat sequences. Phosphorylation of specific serine residues within this CTD is a critical regulatory step that occurs during or just after promoter escape. This phosphorylation helps release RNA polymerase from the promoter, recruit elongation factors, and facilitate downstream steps like RNA processing. This statement is correct.
Statement D: Promoter recognition in bacteria is governed by the sigma factor which binds to the −10 and −35 regions of the promoter followed by recruitment of the RNA Pol II core enzyme to form the holoenzyme.
This statement contains several inaccuracies regarding bacterial transcription. Firstly, in bacteria, the RNA polymerase core enzyme already exists. The sigma factor binds to the *core enzyme* to form the *holoenzyme*. The *holoenzyme* then recognizes and binds to the −10 and −35 promoter regions. The statement reverses the order, suggesting sigma factor binds the promoter first and then recruits the core enzyme. Secondly, it refers to "RNA Pol II core enzyme," which is the eukaryotic RNA polymerase responsible for mRNA synthesis, not the bacterial enzyme. Bacterial RNA polymerase is a single type of enzyme responsible for transcribing all major classes of RNA, with different sigma factors conferring promoter specificity. This statement is incorrect.
Conclusion
Based on the analysis, statements A and C accurately describe aspects of transcription initiation in bacteria and eukaryotes, respectively. Statement B is incorrect regarding the length of eukaryotic abortive transcripts, and Statement D is incorrect about the process in bacteria, specifically the role of the sigma factor and the type of enzyme.
Thus, the combination of all correct statements is A and C.
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 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?
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?
