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 initiation of transcription »

Hypothetical Model for Initiation of Transcription

This simplified view of how transcription is initiated at a particular gene, presents a working hypothesis on how this may occur.

1) A promoter consisting of a TATA-box homology and an uppstream regulatory DNA sequence of a hypothetical gene is shown.
2)  In the cell this DNA segment like all other DNA is forming a chromatin fiber. A transcription factor that has been activated into a DNA binding state by a cellular signaling molecule, is searching along the DNA and is able to find its cognate gene regulatory segment, where it binds sequence specifically to the DNA.
3) This leads to the recruitment of co-activator protein complexes to the chromatin segment that is able to open up the chromatin structure. One such class of factors is the so called SWI/SNF complex. This complex, which is conserved from yeast to man, is able to open up the nucleosome structure during hydrolysis of ATP as the energy source. The other type of chromatin opening complex is the histone acetyl transferase (HAT) which is also conserved from yeast to man.
4) The co-activators will effectuate a chromatin opening, which will also involve neighboring promoter structures.
5) This allows binding of basal transcription factors such as the TATA-box binding complex (which also was shown to contain HAT activity) and other large co-activator complexes. Each one of these various protein complexes contain 10-20 different polypeptides and a lot remains to be learned about their mechanisms of action.
6) RNA polymerase binds either at this stage or perhaps together with some other co-activator complexes.
7)  The RNA polymerase induces strand separation of the DNA and then starts to copy the coding strand of DNA into an RNA molecule.

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