Question

State three characteristics of core promoter elements for RNA polymerase II transcribed genes

Answer 1

Eukaryotes possess three types of RNA polymerase: RNA Pol I, RNA Pol II and RNA Pol III.

Among them, RNA Pol II is responsible for the sythesis of mRNA (messenger RNA) and most of the snRNA (small nuclear RNA).

RNA Pol II core promoter has many characteristic features, three of them are listed below:

(1) Core promoters of RNA Pol II has three major elements: TATA Box, Inr sequence and DPE (Downstream promoter element). Presence of DPE suggest extension of core promoter downstream of the transcription start site.

(2) Most of the RNA Pol II core promoters are of two types: Promoters containing TATA box and TATA-less promoters.

Core Promoters containing TATA-Box has two elements: TATA box (at -25 position i.e., 25 base pairs upstream of the transcription start site (TSS)) and Inr (Initiator) sequence (spans between -3 to +5 position i.e., from 3 base pair upstream to 5 base pair downstream of transcription start site).

TATA-less core promoters has Inr sequence and DPE (present at +28 to +32 in the transcription unit.)

Start Point Inr TATAA -------N20 ----YYCAYYYYY...-- N24 ---- AGAC L ТАТА - Box DPE (Initiator sequence) (Downstream Promoter CORE PROMOTER: Element) CONTAINING TATA - TATA-less CORE PROMOTER Fiq: RNA POI I CORE PROMOTERS.

(3) As RNA Pol II does not have the ability to directly read DNA and bind. therefore, Core promoter elements have binding sites for general transcription factors (also called as basal transcription factors and expressed by symbol TFII X ) which are required to initiate transcription.

RNA Pol II core promoters have low efficiency and therefore also require some activators (another class of transcription factors that bind to proximal promoter) in order to maintain good level of transcription rate.