Genetics Home   Molecular Biology 2nd Edition               Robert F. Weaver
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Chapter 13

  1. See entry #1 under Chapter 11.

  2. We know that several events are involved in gene activation: recruitment of general transcription factors and other elements, either separately or as part of a holoenzyme, chromatin modification, including acetylation, and chromatin remodeling. Agalioti et al. have worked out the order of events during activation of the mammalian interferon- (IFN- ) promoter. The enhanceosome assembles in the enhancer region, which is nucleosome-free. This causes modification and remodeling of a positioned nucleosome that covers the TATA box and transcription initiation site. The first step in this process is recruitment of the GCN5 complex, which acetylates the nucleosome. This allows recruitment of the CBP-polymerase II holoenzyme. The nucleosome acetylation also permits CBP to recruit SWI/SNF, which in turn causes chromatin remodeling. The last step is recruitment of TFIID, which activates transcription. (Agalioti, T, S. Lomvardas, B. Rarekh, J. Y, T. Maniatis, and D. Thanos. 2000. Ordered recruitment of chromatin modifying and general transcription factors to the IFN- promoter. Cell 103:667-678)

  3. We have tended to think of insulators as static blocks to enhancers or silencers. However, new results are showing that insulators can be modulated by other DNA elements. For example, Zhou and Levine show that a newly-discovered element called the promoter-targeting sequence (PTS) in the fruit fly allows the enhancer iab-7 to bypass the insulator Fab8 and activate the Abd-B promoter. (Zhou, J, and M. Levine. 1999. A novel cis-regulatory element, the PTS, mediates an anti-insulator activity in the Drosophila embryo. Cell 99:567-75)

  4. The mechanisms by which insulators exert their effects remain somewhat mysterious. However, recent results have shed further light of this process. For example, Cai and Shen () have shown that one copy of an insulator between an enhancer and a promoter blocks enhancement, but placing an extra copy of the insulator in tandem with the first reverses the effect of the insulator. On the other hand, two copies of the insulator that flank the promoter exert an insulating effect. These results can be explained if the single copy of the insulator between the enhancer and promoter interacts with another element, such as a second copy of the insulator, and isolates the enhancer in a loop. But placing two copies of the insulator between the enhancer and the promoter simply loops out the DNA in between and brings the enhancer closer to the promoter, and makes enhancement easier. (Cai, H.N. and P. Shen. 2001. Effects of cis arrangement of chromatin insulators on enhancer-blocking activity. Science 291:493-95)

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