SITE-SPECIFIC INITIATION OF TRANSCRIPTION BY RNA POLYMERASE-II

RNA polymerase II initiates transcription at specific DNA sequences. S tudies using sequence analysis and molecular genetics suggest a simple and universal model of start-site selection by RNA polymerase II. Two consensus sequences occur at fixed positions in promoters from higher eukaryotes and their viruses: the TATA box around -30 and the initiat or at the start site of transcription. Both consensus sequences functi on as positioning elements that control site-specific initiation. As a first step during initiation, the basal transcription factor TFIID bi nds to the TATA box; regulatory transcription factors can tether TFIID to promoters without a consensus TATA box. TFIID then directs the ass embly of other basal transcription factors and RNA polymerase II into a preinitiation complex. Finally, RNA polymerase II searches for the b est match to the initiator consensus about 30 base pairs downstream of the TATA box to select the exact start site. The transcriptional acti vity of a start-site sequence generally correlates with its similarity to the initiator consensus, suggesting that there is only one type of initiator.