The Eucaryotic Transcription Apparatus

Three types of RNA polymerases exist for the transcription of eucaryotic genes, each of which transcribes a certain class of genes. All three enzymes are characterized by a complex subunit structure. 

1 Structure of the Transcription Start Site and Regulatory Sequences 

Most promotors of of class II genes share three common features (Fig. 1.20A) the transcriptional start site, the TATA box, and sequences bound by transcriptional regulators. A typical core promotor containing the start site and the TATA box encompasses ca. 100 bp. 

The TATA box or an initiation element are structural elements which define a minimal promotor required for recruiting the appropriate RNA polymerase and for initiation of transcription. In higher eucaryotes, the TATA box is often, though not always, ca. 30 bp from the transcription start site. The initiation element includes sequences in the immediate vicinity of the transcription start site. Not every eucaryotic promotor 

Regulatory sequences can also be located far from the promotor and are called enhancers if the cognate DNA-binding protein is a transcriptional activator. They influ- 

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