In eucaryotic system, the polymerases require the prior assembly of general transcription factors at the promoter. These factors assemble in a particular order, beginning with the binding of TFIID to the TATA box, a DNA sequence found just upstream of most eucaryotic RNA polymerase staaart sites. The ordered assembly of general transcription factors provides several steps at which the initiation of transcription can be regulated, and many eucaryotic gene regulatory proteins are thought to work by faciliating or hindering the assembly process.
TFIID is composed of many subunits; that responsible for recognizing the TATA sequence is called TBP (TATA- binding protein). Next, TFIIB enters the complex. The other factors, along with RNA polymerase II, are added in turn. Biochemical studied indicate that the interaction of TBP with TFIIB is critical for activated transcription in vitro, in order to probe the importance of the TBP-TFIIB interaction for transcription in vivo, measuring the response using two hybrid system in yeast . In addition, an altered-specificity TBP-TFIIB interaction was designed with mutation and linked in sequence to an altered-specificity TATA box-TBP interaction to study how TBP and TFIIB function together to support transcription.
Activators are thought to work by facilitating the assembly process. The altered specificity of TBP-TFIIB demonstrates the interaction influences the stimulation of transcription by activators. But one activator, Sp1, activated transcription independently of this interaction. So selective use of TBP and TFIIB in activator function revealed by a TATA-TBP-TFIIB array with altered specificity.
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