13. The following statements are related to transcription in bacteria and 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 a-amanitin to RNA polymerase II 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 octamers during transcription
    Which of the following combinations of statements represents all correct statements?
    (1) A, B and C                                              (2) A, B and D                         (3) B, C and D            (4) B and C only

Analysis of Statements

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

• In bacteria, during transcription initiation, the sigma factor (e.g., sigma 70) recognizes the -10 and -35 promoter elements.
• For melting, specifically at the -10 element, DNA base flipping occurs. Studies have shown that two bases (typically -11A and -7T on the non-template strand) flip out of the DNA helix and are captured in hydrophobic pockets within the sigma factor. This “flip-and-capture” mechanism helps to stabilize the melted DNA (transcription bubble).
• This statement is correct.

B. Binding of a-amanitin to RNA polymerase II permits entry of nucleotides into RNA pol II active site and synthesis of RNA, but prevents translocation.

• Alpha-amanitin is a potent toxin that specifically inhibits eukaryotic RNA polymerase II (and to a lesser extent, RNA polymerase III).
• Research shows that alpha-amanitin binds to RNA Pol II and does allow initial nucleotide entry and phosphodiester bond formation (synthesis of short RNA fragments). However, its primary effect is to prevent the translocation of the RNA polymerase along the DNA template, effectively stalling elongation.
• This statement is correct.

C. RNA polymerase I can use upstream promoters with 3 consensus sequences, as well as internal promoters having a bipartite structure

• RNA polymerase I (Pol I) primarily transcribes ribosomal RNA (rRNA) genes in eukaryotes.
• In higher eukaryotes, Pol I promoters typically consist of two main elements: an upstream control element (UCE) and a core promoter element. These are generally located upstream of the transcription start site.
• RNA polymerase III (Pol III), not Pol I, is known for recognizing internal promoters with a bipartite structure (e.g., in tRNA genes and 5S rRNA genes).
• This statement is incorrect.

D. FACT is associated with RNA polymerase during transcriptional elongation and helps displace histone octamers during transcription

• FACT (FAcilitates Chromatin Transcription) is a histone chaperone protein found in eukaryotes.
• It is indeed associated with RNA polymerase II during transcriptional elongation.
• Its function is to facilitate transcription through chromatin by displacing H2A-H2B histone dimers from nucleosomes ahead of the elongating polymerase and then reassembling them behind the polymerase, thus making the DNA accessible for transcription and maintaining chromatin integrity. It doesn’t displace the entire histone octamer (H2A-H2B, H3-H4).
• This statement is correct.

Summary of correctness:

• A: Correct
• B: Correct
• C: Incorrect
• D: Correct

Therefore, the combination of correct statements is A, B and D.

The final answer is (2)​

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