Question

In transcription, what structural changes indicate the transition from initiation to elongation

Answer 1

Answer=

In transcription, the structural changes indicate the transition from initiation to elongation are as follows:

Transcription is the first step in gene expression and is the major target of reulation.

-Can be divided into three distinct phases i.e. Initiation,elongation and termination.

structural changes indicate the transition from initiation to elongation in bacteria =

- In eubacterial species,transcription of all genes is mediated by a core RNAP complex,typically a 5-subunit ($\alpha$2$\beta \beta$'$\omega$ ) enzyme.

-However,in order to recognize promoter DNA sequences ,this core enzyme must associate with a sigma factor to form RNAP holoenzyme.

-Initiation occurs at a site that is fixed distance from the $\sigma$ recognition sequences.

-Eubacterial species typically contain multiple $\sigma$ factors that form distinct classes of RNAP holoenzymes that recognise different promoters sequences and reguate distinct classes of genes.

-steps in transcription initiation are

1.preinitiation closed complex formation at the promoter by RNAP holoenzyme,containing $\sigma$ factor.

2.DNA is unwound around the transcription start site to form an open complex.

3.Abortive synthesis of 2-15 nt RNAs requiring DNA "scrunching".

4.Promoter escape istypically associated with loss of $\sigma$ factor.

structural changes indicate the transition from initiation to elongation in eukaryotes =

-Eukaryotic cells contain 3 nuclear RNA polymerases,with RNA polymerase II (polII) responsible for transcribing all mRNAs and numerous non-coding RNAs.

-Pol II ,a 12 subunit enzyme with many similarities to bacterial RNAP,does not recognize promoter DNA by itself,but rather a part of the basal pol II machinary that includes general transcription factors i.e.TFIIA,B,C,D.E.F,H.

-These factors do not associate with elongating Pol II,and hence rapidly dissociate from Pol II during the transition between initiation and elongation.

- number of factors e.g.FACT,Spt 4,Paf 1 and TREX complexes,Spt6,travel with elongating Pol II throughout the coding region.

- Eukaryotic Pol II must contend with nucleosomes that inhibit both initiation and elongation.

- Open complex formation requires the helicase activity of TFIIH,and promoter escapes does not coincide with abortive initiation.

-Escape from promoter-proximal pauses is associated with phosphorylation at serine 2 of the C-terminal domain of the largest Pol II subunit by TEFb.