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and work as chemotherapeutic agents or sensors has generated a great deal of excitement. AT-rich sequences have been particularly challenging targets for zinc finger–domain approaches. Noyes et al. have turned to the other large category of sequence-specific transcription fac- tors, homeodomain proteins. They have carried out a comprehensive survey of the breadth of specificity of the 84 known Drosophila homeodomains that function independently of other DNA-binding domains. The relations between particular amino acid residues and preferred binding sequence were com- plex, but general determinants were assigned according to whether they cooper- ated or competed in binding character, leading to predictions for the binding specificity of roughly 75% of the homeodomains in the human genome and allowing them to modify Engrailed to exhibit a binding specificity resem- bling that of TGIF even though these proteins share only 25% amino acid identity. On the basis of their analysis, the authors have created a Web-based tool that supports the prediction of specificities for homeodomains from other organisms. — BJ Cell 133 , 1277 (2008). 4 JULY 2008 VOL 321 SCIENCE www.sciencemag.org 16 CREDITS (TOP TO BOTTOM): (ILLUSTRATION) N. KEVITIYAGALA/ SCIENCE ; NOYES ET AL., CELL 133 , 1277 (2008) EDITORS’ CHOICE CELL BIOLOGY Like Ps in a Pod In the budding yeast , the 26 S proteasome degrades many proteins involved in cell-cycle pro- gression and thus is essential for cell proliferation. In actively growing yeast, 80% of the 26 S protea- some, which comprises a 20S core particle and a 19 S regulatory particle, is localized inside the nucleus. In quiescent cells, proteasome proteolytic activity decreases and correlates with release of the regulatory particle, but the fate of the disassem- bled subcomplexes remains unclear. Laporte et al. found that when cells exhausted their carbon source and entered quiescence, sub- units from the 20 S and 19 S particles colocalized into cytoplasmic foci termed proteasome storage granules (PSGs). Consistent with the proposal that PSGs act as storage depots, refeeding the cells resulted in rapid relocalization of proteasomes into the nucleus and did not require de novo protein synthesis. Other macromolecular assemblies trig- gered by quiescence have been described, such as
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This note was uploaded on 08/22/2011 for the course EGM 4313 taught by Professor Mei during the Spring '08 term at University of Florida.

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