Breaking the Silence

摘要

The genomes of eukaryotic cells are not composed of free DNA but exist as chromatin in which the DNA is associated with octamers of histone proteins called nucleosomes. The structure of chromatin varies throughout the genome, providing a means to regulate access of the transcriptional machinery to the underlying genes. In an extreme case, chromatin adopts a condensed structure termed hete- rochromatin in which genes are less accessible and frequently transcriptionally silent. Uncondensed chromatin (euchro-matin) is much more accessible than hete-rochromatin and contains the majority of actively expressed genes. To regulate gene expression, cells adopt several different strategies to alter chromatin structure. These include the posttranslational modification of histones (for example, by acetyla-tion), the incorporation of variant histone proteins into nucleosomes, and adenosine triphosphate (ATP)-dependent chromatin remodeling by protein complexes related to the yeast Swi2/Snf2 ATPase. Three recent reports, including one by Mizuguchi et al. (1) on page 343 of this issue, indicate that these different strategies may be more inti-, mately related than previously appreciated.