Q.18 Which of the following statements related to transcription in bacteria/eukaryotes are correct?

1. 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.
2. Binding of α-amanitin to RNA polymerase II permits entry of nucleotides into RNA polymerase II
   active site and synthesis of RNA, but prevents translocation.
3. RNA polymerase I cannot bind to upstream promoters.
4. FACT is associated with RNA polymerase during transcriptional elongation and helps displace one
   histone octamer during transcription.

Choose the correct answer from the options given below:

1. (A), (B) and (D) only
2. (A), (B) and (C) only
3. (B), (C) and (D) only
4. (B) and (D) only

   Statements (B) and (D) are correct regarding bacterial and eukaryotic transcription mechanisms.

   Statement Analysis

   (A) Incorrect. In bacterial transcription initiation, sigma factor recognizes the promoter (-10/-35 boxes), and melting involves unwinding ~14 bp via RNAP’s wedge without base flipping by sigma; base flipping occurs later in some repair contexts, not primary sigma.

   (B) Correct. α-Amanitin binds RNA Pol II’s bridge helix, blocking translocation after nucleotide addition, trapping the enzyme post-catalysis but halting forward movement and RNA extension.

   (C) Incorrect. RNA Pol I binds upstream promoters (core + upstream elements via SL1/UBF) to transcribe rRNA; it requires these for stable PIC formation.

   (D) Correct. FACT (Facilitates Chromatin Transcription) associates with elongating RNAP during eukaryotic transcription, aiding H2A-H2B eviction to allow passage and partial histone displacement.

   Correct Option

   (B) and (D) only matches the accurate statements on RNAP mechanisms and chromatin dynamics.

   Bacterial eukaryotic transcription differs in initiation, elongation, and regulation, critical for GATE Life Sciences PYQs testing RNAP specifics like alpha-amanitin inhibition and FACT’s chromatin role.

   Bacterial Initiation Details

   Bacterial RNAP holoenzyme (sigma70) binds promoters; melting uses rudder/wedge for bubble formation without sigma-induced base flipping, enabling rapid promoter escape.

   Eukaryotic RNAP II Inhibition

   Alpha-amanitin specifically inhibits RNAP II by binding the funnel, allowing nucleotide entry but preventing post-addition translocation, stalling elongation.

   RNA Pol I Promoter Binding

   Unlike Pol III (internal promoters), Pol I requires upstream core/upstream elements with UBF/SL1 for rDNA transcription initiation.

   FACT in Elongation

   FACT travels with RNAP II, destabilizing H2A-H2B dimers during elongation to displace nucleosomes temporarily, reforming behind for chromatin integrity.