Characterization of the Isw1a and Isw1b ATP-dependent chromatin remodeling complexes from the budding yeast, Saccharomyces cerevisiae
Vary, James Corydon, 1971-
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Members of the ISWI class of ATP-dependent chromatin remodeling complexes have been found in nearly every eukaryote studied, suggesting important functions that have been conserved throughout evolution. The budding yeast, S. cerevisiae, has two homologs of the conserved ISWI ATPase, Isw1p and Isw2p. Although these complexes share similar biochemical activities to remodel chromatin, their in vivo roles are unclear. Chromatographic separations and immunoprecipitations of Isw1p and its associated proteins revealed that Isw1p forms two distinct complexes in vivo. Each complex interacts with DNA and nucleosomes in vitro, and can reposition nucleosomes in an ATP-dependent manner. In vivo , these complexes have overlapping, yet distinct roles in transcriptional regulation, and each shows different genetic interactions with other genes. To identify factors that operate in parallel to these Isw1-containing complexes, deletions of the histone H3 and H4 N-terminal tails were made. ISW1 genetically interacts with both histone mutants at elevated temperatures suggesting that a histone tail-dependent pathway operates in parallel to one or both Isw1 complexes. Mutational analysis of histone H4 revealed that ISW1 interacts with the four acetylatable lysines on the H4 N-terminal tail, closely matching the substrate preference of the NuA4 histone acetyltransferase complex. Mutants of NuA4 components have ISW1-dependent phenotypes similar to the histone mutants at elevated temperatures and on media containing the drug rapamycin. The specificity of these interactions strongly suggests a role for TOR (target of rapamycin)-signaling in a parallel pathway to one or both Isw1 complexes. The identification of the nature of this interaction will yield clues as to the functions of Isw1 complexes in vivo that may be extended to higher eukaryotes.