Question

Describe the general strategies adopted for initaion of transcription in eukaryotes. In your answer consider the roles and initiation requirements of the three eukaryote RNA polymerases; pol I, pol II and pol III

Answer 1

Eukaryotes have three different RNA polymerases : RNA Pol I, RNA Pol II and RNA Pol III.RNA Pol I is responsible for synthesizing most of the rRNA , pol II synthesizes mRNA and most of the snRNA , and pol III transcribes tRNA , some small nuclear RNA genes and the 5S rRNA gene.

Initiation of transcription in eukaryotes requires transcription factors and the enzyme RNA polymerase. The transcription factors create a structure at the promoter to provide the target that is recognized by the RNA polymerase.

RNA polymerase II - The core promoter of pol II consists of two segments - TATA box or -30and the initiator sequence (Inr) located around nucleotide sequence +1. Pre- initiation complex is a complete set of GTF( general transcription factors) and polymerase bound together at the promoter. Formation of pre-initiation complex beings with the binding of GTF " TFIID" to the TATA element. TFIID is a multi subunit complex and its TBP (TATA- binding protein) component binds to the TATA DNA sequence. The other subunits of this complex are called TAFs, for TBP- associated factors and some TAFs recognize other core promoter elements like Inr, DPE. Once TBP is bound to the DNA, it distorts the TATA sequence extensively. The resulting TBP-DNA complex then recruits polymerase and other GTFs ( TFIIE, TFIIH) to the promoter. Once assembled at the promoter, Pol II leaves the preinitiation complex upon addition of the nucleotide precursors required for RNA synthesis.

RNA polymerase I- The promoter of Pol I comprises two parts - the core element and the UCE (upstream control element). The transcription factors of Pol I are SLI and UBF. SL1 comprises TBP and three TAFs specific for Pol I transcription. This complex binds to the core element. SL1 binds DNA only in the presence of UBF. UBF binds to UCE, bringing in SL1 and stimulating transcription from the core promoter by recruiting Pol I.

RNA polymerase III - Pol III promoters come in various forms ,some Pol III promoters ( e.g. those for tRNA genes ) consist of two regions - Box A and Box B ,separated by a short element ; others contain Box A and Box C ( e.g. the 5S rRNA gene) .Like Pol II and Pol I, transcription by Pol III requires transcription factors in addition to polymerase. The factors are TFIIIB and TFIIIC for the tRNA genes and those plus TFIIIA for the 5S rRNA genes.