Question

Describe how to control transcriptional initiation occurs in both PROKARYOTES and EUKARYOTES.

Word bank:

promoter   TATA-Binding Protein (TBP)   RNA polymerase + sigma factor

enhancer    TATA-box       gene-specific transcription factors

RNA polymerase + GTFs   phosphodiester bonds    -10 and -35 consensus sequences

mediator protein   +1 Transcriptional Start Site (TSS)   deoxynucleoside triphosphates (dNTPs)

template DNA RNA transcript.

Answer 1

Transcription is the process of synthesis of RNA from DNA. It included 3 major steps - initiation, elongation and termination. This steps are fulfilled slightly differently in prokaryotes and eukaryotes.

To talk specifically about transcription initiation, in prokaryotes, initiation occurs at a sequence upstream of the coding sequence. This region is called the promoter. Two conserved sequences in this region, -35 and -10, are especially important. These are with respect to the transcription start site ( TSS; +1)The -35 sequence is TTGACA to which the sigma factor binds to. The sigma factor acts as the 5th subunit of the RNA polymerase and provides specificity for the transcription start site. On the other hand the -10 sequence is TATAAT (TATA Box) and is the site where the DNA template unwinds. The RNA polymerase is , hence, recruited at the promoter with the aid of the sigma factor (5th subunit). The remaining 4 subunits, namely $\alpha \alpha’ \beta \beta’$ '$\beta and \beta$' , form the core enzyme and are involved in the catalysis of the process addition complementary nucleotides to the template strand and simultaneously forming a phosphodiester linkage (5'P to 3'OH) between the 2 adjacent nucleotides.

In eukaryotes on the contrary, the transcription initiation varies in terms of the transcription factors involved - TBP (TATA Box binding protein) , TFIID, TFIIA, TFIIB, TFIIE, TFIIA, TFIIH- in that order which together recruit the RNA POLYMERASE there.