Question

Describe briefly the structure and subunits of prokaryotic and eukaryotic RNA polymerases. Describe in detail at least three ways that prokaryotic and eukaryotic transcription differ from each other.

Answer 1

Prokaryotic RNA polymerases

1. Only one type of RNA polymerase present. Main function is to synthesis RNA from DNA template
2. Each core  RNA polymerase contains five subunits
3. Two alpha (α) subunits, one a beta (β) subunit and a beta prime subunit (β′) and an omega (ω) subunit
4. Weight of two alpha subunits is 36 kilo Dalton (kDa),
5. Weight of a beta (β) subunit is150 kDa
6. Weight of beta prime subunit is155 kDa
7. "clamp-jaw" or  "crab claw" model best describes the structre of  the core RNA polymerase complex
8. RNA polymerase holoenzyme is formed when a sigma (σ) factor binds to it,

Eukaryotic RNA polymerases

1. Five different types of RNA polymerases are found in Eukaryotes
2. RNA polymerase I transcribes only rRNA
3. RNA polymerase II transcribes Pre mRNA or precursors of mRNAs
4. RNA polymerase II also transcribes snRNAs and microRNAs
5. tRNAs and 5S rRNA is synthesized by RNA polymerase III
6. Functions of RNA polymerase IV and V are not well understood. These are found only in plants. Evidences shows that these are involved in synthesizes small interfering RNA
7. RNA polymerase I is a 590 kDa enzyme with 14 protein subunits
8. RNA polymerase II A 550 kDa enzyme with12 subunits
9. RNA polymerase III is the product of some house keeping genes. It is the largest eukaryotic RNA polymerase with 17 subunits and a molecular weight of ∼0.7 MDa

Difference between prokaryotic and eukaryotic transcription

Prokaryotic transcription	Eukaryotic transcription
Location of transcription is cytoplasm	Location of transcription is nucleus
Transcription and translation are coupled	Transcription and translation occur at different times
Polycistronic mRNA is transcribed	Monocistronic mRNA is transcribed
Termination depends on Rho factor	Termination do not depend on Rho factor