Eukaryotic transcription factors
In eukaryotes RNA polymerase alone cannot perform transcription, in addition certain group of proteins called transcription factors are also required to initiate the process. RNA polymerase and transcription factors together forms a stable complex at transcription initiation site by binding to TATA region of promotor site. Transcription factors are divided into two types such as i) general transcription factors and ii) sequence specific transcription factors. General transcription factors(GTF) are required to transcribe all RNAP-II dependent genes. Where as sequence specific transcription factor(SSTF)regulate transcription. There are 6 GTF required to transcribe RNAP dependent genes. They are TF-IIA, TF-IIB, TF-IID, TF-IIE, F and H.
Transcription factors for RNAP-II
First TFIID transcription factor recognize TATA sequence of promotor site. It has two components namely TATA binding protein(TBP) and TBP associated factor(TAF). TBP recognize TATA box where as TAFs recognize various promoters. The binding of TFIID is fallowed by the joining of TFIIA which is activated by TBP. This complex protect a region later binding of TFIIB occurs on both upstream and downstream region(-10, +10) on DNA. It binds adjacent to TBP to form DB complex. The TFIIF has two subunits, the larger subunit is involved in melting of DNA at initiation point and smaller sub unit binds to RNAP. Two more factors TFIIE and TFIIH get bind to the transcription complex. The ATPase and helicase activity of TFIIH helps in the movement of RNAP-II from the initiation site. Transcription begins with RNAP-II movement fallowed by release of all transcription factors from the DNA.
Transcription factors for RNAP-III
RNAP-III synthesizes tRNA and 5sRNA. It requires few transcription factors like TFIIA and TFIIB are required for tRNA synthesis. The TFIIIB is the only factor recognized by RNAP-III to initiate transcription other assembly factors like TFIIIA and TFIIIC help in binding of TFIIIB to right location of DNA. The TBP of TFIIIB helps in recognition of TATA box for initiation of transcription.
Transcription factors for RNAP-I
RNAP-I synthesize rRNA. It requires two helping factors such as i) UB-F1 that binds to GC rich regions of promoter and ii) SL1 which bind to the DNA. The RNAP-1 binds to the promotor only after the binding of these two helping factors and then transcription takes place.