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It takes teamwork to modify chromatin

The organization of constituents in a large chromatin-modifying complex reveals specific roles [Also see Report by Morgan et al.] To participate in the choreography of signals that occur on the chromosome during gene activation, many multisubunit chromatin-modifying complexes contain more than one enzyme that chemically alter histones, the protein constituent of chromatin. These complexes themselves often contain modules or clusters of subunits dedicated to specific functions of the complex (1). The Spt-Ada-Gcn5 acetyltransferase (SAGA) complex is a good example of this organization. It is highly conserved from yeast to humans, contains up to 20 subunits, and is about 2 MDa in size. SAGA contains several functional modules, two of which have enzymatic activities and others that mediate SAGA interactions with proteins that control transcription (2). Hence, teamwork between these modules is crucial for the steps leading to transcription initiation and its transition to RNA elongation (3). Even within each module, teamwork between individual subunits is necessary to promote accurate enzymatic activity. On page 725 in this issue, Morgan et al. (4) provide structural and biochemical data that nicely illustrate cooperation between subunits within the module that contains a histone deubiquitinase, and between this module and the remainder of the SAGA complex.