The “Dark Side” of Chromatin Minireview Remodeling: Repressive Effects on Transcription

octamer relative to the DNA. The movement of a histone octamer along the DNA must involve the breaking and reestablishment of histone–DNA contacts within the nucleosome, but exactly how the histone–DNA contacts are altered is not known. In theory, the ATP-driven trans-location of a chromatin remodeling complex along the The packaging of the eukaryotic genome into chromatin DNA could directly break the histone–DNA contacts permits dynamic and broad-ranging changes in gene within a nucleosome. Alternatively, remodeling com-expression. The basic unit of chromatin is the nucleo-plexes may function in an indirect manner to alter his-some, which comprises ‫002ف‬ bp of DNA wrapped about tone–DNA contacts, such as by the induction of a gen-two turns around an octamer of core histone proteins eral conformational change in the DNA and/or the (Luger et al., 1997; reviewed by Kornberg and Lorch, histone octamer. 1999). The assembly of a gene into chromatin generally How the histone octamers move relative to the DNA represses transcription by inhibiting the binding and is a matter of conjecture, and several different models function of key components of the transcriptional appa-have been proposed (for animations, see: http://www. ratus. To facilitate the function of such factors in the dundee.ac.uk/‫ف‬taowenhu/Toh_show.htm). Most of these context of chromatin, there are a variety of ATP-utilizing current models have, however, a related theme of oc-chromatin remodeling factors whose fundamental func-tamer movement, which is as follows. First, DNA dissoci-tion is the mobilization of nucleosomes via the alteration ates from the histone octamer at one edge of the nucleo-of histone–DNA contacts. some. Then, a neighboring stretch of DNA associates Chromatin remodeling complexes have been gener-with the histone octamer (to reestablish the previously ally studied as factors that promote gene activation (recently reviewed by Travers, 1999; Varga-Weisz and Becker, 1999). In this minireview, we will first provide a brief overview of chromatin remodeling. Then, we will turn toward the dark (i.e., repressive) side of these chro-matin remodeling factors and describe recent studies indicating that ATP-driven chromatin remodeling factors facilitate not only transcriptional activation, but also repression. Lastly, we will compare the processes of chro-matin remodeling and nucleosome assembly. Notably, these two related processes are both essential for the proper regulation of gene expression. A Growing Family of ATP-Utilizing Chromatin Remodeling Factors Chromatin remodeling factors comprise an ATPase sub-unit along with other polypeptides that are responsible for the regulation, efficiency, and functional specificity of each complex (Table 1). The ATPase …