Question

Regarding transcription in prokaryotes:

I. For each stage listed below, select the holoenzyme of RNA polymerase if it is required for proper completion of the stage or select the core enzyme if it can accomplish this step without the rest of the enzyme.

1.Elongation - a.Core Enzyme or b. Holoenzyme
2. Initiation - a. Holoenzyme or b. Core Enzyme
3. Template Binding - a.Holoenzyme or b. Core Enzyme
4. Termination - a. Holoenzyme or b. Core Enzyme

II.What component/protein/subunit is present in the holonenzyme but is not present in the core enzyme?
III. What component/protein/subunit is sometimes required for proper termination of transcription?

Answer 1

Transcription in prokaryotes

Step-by-step explanation:

RNA polymerase is a multisubunit enzyme consisting of five different polypeptides called α,β,β',ω,σ (holoenzyme)

Holoenzyme is divided into 2 subunits: σ subunit(promoter recognition and initiate transcription);core enzyme(consists of α,β,β' and ω subunits,they all catalyze transcription)

1. Elongation-b.Core Enzyme

Once RNA Polymerase successfully synthesize a short stretch of RNA,a protein binds to the elongating RNA Polymerase

RNA transcript synthesized by RNA Polymerase continuously dissociates from DNA template as well as transcription proceeds

Only 8-10 base pairs long RNA-DNA complementary is maintained in transcription bubble

2. Initiation-a. Holoenzyme b.Core Enzyme

Transcription initiated when core RNA Polymerase binds with promoter with the help of σ subunit

σ subunit act as a prokaryotic transcriptional initiation factor

σ factor is released as RNA Polymerase moves

3. Template Binding-a. Holoenzyme or b.Core Enzyme

The σ factor associates with core enzyme to form holoenzyme

The Holoenzyme binds with promoter to form closed binary complex

Unwinding of double strands occurs creating an open complex

Addition of first nucleoside triphosphate to form ternary complex

σ factor is released as RNA Polymerase moves

4. Termination-b.Core Enzyme

Transcription termination occurs at the end of gene and is always guided by transcribed RNA molecule because transcription signal present in RNA

Termination occurs by 2 methods:

• ρ dependent termination- ρ protein unwinds the RNA-DNA hybrid(using RNA-DNA helicase activity) and release RNA transcript
• ρ independent termination-here formation of hair pin structure occurs which destabilize the ribouracil-deoxy adenine complementary and release RNA transcript