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Chromatin not only serves as a packaging material, but also functions as a platform for integrating signals that act upon the genome. Indeed, chromatin is a dynamic macromolecular structure that can be dramatically altered in many ways to facilitate the different transactions at the genome. Examples of such alterations are relocalization of genomic loci within the nucleus upon transcriptional activation or induction of DNA damage, adding or removing post-translational modifications on histones or other chromatin-binding factors, or altering the basic organization of chromatin by moving or removing nucleosomes, i.e. modifying the occupancy of histone octamers. New insights into the scope and mechanisms of chromatin dynamics have recently been obtained by the development of novel techniques to visualize chromatin protein mobility and stability. Here we discuss the developments in this area, with special emphasis on histone exchange, which we define as the replacement of histone proteins without a prerequisi...

Sir3, a component of the transcriptional silencing complex in the yeast Saccharomyces cerevisiae, has an N-terminal BAH domain that is crucial for the protein's silencing function. Previous work has shown that the N-terminal alanine residue of Sir3 (Ala2) and its acetylation play an important role in silencing. Here we show that the silencing defects of Sir3 Ala2 mutants can be suppressed by mutations in histones H3 and H4, specifically, by H3 D77N and H4 H75Y mutations. Additionally, a mutational analysis demonstrates that three separate regions of the Sir3 BAH domain are important for its role in silencing. Many of these BAH mutations also can be suppressed by the H3 D77N and H4 H75Y mutations. In agreement with the results of others, in vitro experiments show that the Sir3 BAH domain can interact with partially purified nucleosomes. The silencing-defective BAH mutants are defective for this interaction. These results, together with the previously characterized interaction between the C-terminal region of Sir3 and the histone H3/H4 tails, suggest that Sir3 utilizes multiple domains to interact with nucleosomes.