Eukaryotic transcription initiation (Saccharomyces cerevisiae)

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In eukaryotes, RNA polymerase, and therefore the initiation of transcription, requires the presence of a core promoter sequence in the DNA. RNA polymerase is able to bind to core promoters in the presence of various specific transcription factors. The most common type of core promoter in eukaryotes is a short DNA sequence known as a TATA box. The TATA box, as a core promoter, is the binding site for a transcription factor known as TATA binding protein (TBP), which is itself a subunit of another transcription factor, called Transcription Factor II D (TFIID). After TFIID binds to the TATA box via the TBP, five more transcription factors and RNA polymerase combine around the TATA box in a series of stages to form a preinitiation complex. One transcription factor, DNA helicase, has helicase activity and so is involved in the separating of opposing strands of double-stranded DNA to provide access to a single-stranded DNA template. However, only a low, or basal, rate of transcription is driven by the preinitiation complex alone. Other proteins known as activators and repressors, along with any associated coactivators or corepressors, are responsible for modulating transcription rate. Source: Wikipedia

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Reference: http://www.web-books.com/MoBio/Free/Ch4E.htm