Structure of the Transcription Start Site and Regulatory Sequences

1 Structure of the Transcription Start Site and Regulatory Sequences 

The promoter activity of class II genes is guided primarily by three structural elements (Fig. 1.30)  

The TATA box and/or an initiation sequence are structural elements which define a minimal promoter from which in vitro transcription can be initiated. A classical TATA box is often, though not always, ca. 30bp from the transcription start site. The initiation sequence includes sequences in the immediate vicinity of the transcription start site. The TATA box and initiation sequence are sufficient for the formation of a basal transcription apparatus composed of general initiation factors for transcription and RNA polymerase II (see Fig. 1.31). 

Not every eucaryotic promoters possesses a TATA box. For promoters devoid of a TATA box, the initiation sequence is determining for promoter selection and formation of the pre-initiation complex. 

An increase in the basal transcription activity originating from the promoter requires regulatory DNA sequences. These sequences can be proximally or distaUy located and serve as binding sites for transcriptional activators. The cis-acting DNA elements can be found near the promoter in either orientation. They are often termed upstream activating sequences (UAS). Regulatory sequences can also be located far from the promoter. Their effect is independent of their orientation and they are known as enhancers. 

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