The TATA box is surrounded by G-C rich sequences and is comparable to Pribnow box of prokaryotes (see Regulation of Gene Expression 3. A Variety of Mechanisms in Eukaryotes also). Further upstream is another sequence called CAAT box, which, being necessary for initiation, is conserved in some promoters (β globin gene), but is not necessary in some other genes (thymidine kinase gene of herpes virus). This sequence lies between -70 and -80 base pairs and includes the following.
Another sequence called GC box (GGGC GG) is found in one or more copies at -60 or -100 bp upstream in any orientation in several genes. It has been shown that CAAT and GC boxes determine the efficiency of transcription, while TATA box aligns RNA polymerase at proper site, with the help of TFIID and other transcription factors. (See later for transcription factors).
Eukaryotic promoters often also possess elements located 100 or 200 base pairs upstream, which interact with proteins other than RNA polymerase and thus regulate the activity of promoter. These elements, called enhancers, also called upstream activation sites or UAS, can be moved several hundred or even thousands of base pairs upstrearrj,or downstream without alteration of their activities. These enhancers may lead upto a 200 fold increase in transcription rate of an affected gene, hence the name. There are other regulatory elements known as silencers, which repress gene expression. As shown by Alexander Johnson (San Francisco), silencers, like enhancers,can function at great distances from genes they repress. They are sites for binding of proteins just like enhancers. The location of different promoters and enhancers is shown in Fig. 32.13. More details about enhancers and their role in gene expression are given in Regulation of Gene Expression 3. A Variety of Mechanisms in Eukaryotes. (Silencers are found in yeast and repress expression of HML and HMR loci involved in switching of mating types; consult Regulation of Gene Expression 3. A Variety of Mechanisms in Eukaryotes).
One would be interested in knowing how RNApolymerase III, attached to +55 to +80 bp sequence, can start transcription from the start point. It is believed that the enzyme is big enough to occupy start point and the promoter region simultaneously and thus may start transcribing at the start point without difficulty.
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