General transcription factor
General Transcription Factors can be abbreviated to GTFs. The title covers all transcription factors which are involved in allowing eukaryotic RNA polymerase to recognise the promoter regions of a gene. Their function is very similar to that of σ-factors in the recruitment of bacterial RNA polymerase to the promoter consensus sequences found upstream of bacterial genes. The GTFs assemble around the promoter region of a gene to form the PIC (Pre-Initiation Complex) which is equivalent to the closed complex for bacterial transcription.
This GTF is key to the assembly of the PIC. IID has two domains called TATA binding protein (TBP) and TBP associated factors (TAFs). The TBP recognises the TATA box in the core promoter region of the gene and binds to it. This is a key recognition stage. TAFs promote the interaction of IID with the basal promoter and they also act as a coactivator that interacts with activators.
This GTF is recruited to the PIC assembly by IID to stabilise the complex. Once transcription elongation has begun and the promoter has been cleared IID and IIE are likely to remain bound to the DNA TATA box, ready to initiate the assembly of the next PIC.
This transcription factor is recruited to the PIC assembly once IIA has bound to the complex. IIB binds to the complex so that is located close to the transcription initiation start site. IIB is key to the recruitment of the RNA polymerase II to the PIC (obviously RNA polymerase II is crucial to the correct formation of the PIC).
This transcription factor is likely to be bound to the RNA polymerase when it gets recruited, so it is added to the complex once IIB has recruited the RNA polymerase II.
This transcription factor is recruited after IIF and before IIH.
This is another central GTF to the PIC assembly. It possesses both kinase and helicase properties. The helicase property allows the transcription factor to cause localised DNA melting so that transcription can be initiated (as the RNA polymerase II now has a ssDNA template to work from). It binds to the transcription start site when recruited and is the last GTF to be recruited for the assembly of PIC.
What happens to each GTF once transcription has begun?
TFIIA and TFIID are likely to remain bound to the DNA at the TATA Box as this leaves them prepared for the assembly of the next PIC to speed up the initiation of transcription.
TFIIB, TFIIH and TFIIE tend to disassociate from the PIC once transcription has begun and promoter clearance has occurred so that they can be recycled for the next PIC assembly.
TFIIF remains bound to the RNA polymerase II as it translocates along the DNA.