Eukaryotic Chromatin Structure and Gene Regulation

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Our group has continued studies of chromatin structure and the regulation of eukaryotic gene expression. We have made significant progress towards understanding the mechanism of ATP-dependent chromatin remodeling by Drosophila NURF (Nucleosome Remodeling Factor). To elucidate the biological significance of NURF-induced nucleosome sliding, we have isolated and extensively characterized mutations for Drosophila nurf301. We confirm that NURF is required for transcriptional regulation of a variety of genes important for development and cell signalling in vivo. Stikingly, mutants in NURF subunits exhibit neoplastic transformation of larval blood cells that induces melanotic tumor formation. These results reveal that NURF is required not only for proper regulation of genes involved in the stress response and development, but also for tumor suppression. We have traced this disease phenotype to a dysregulation of the JAK-STAT signalling pathway, and are conducting a molecular analysis of the affected target genes. We are also making excellent progress on studies of INO80 and SWR1 complexes, which are new members of the SWI2/SNF2 superfamily of chromatin remodeling enzymes. We have conducted genetic and biochemical experiments to elucidate the mechanism and physiological functions of the complexes. The INO80 complex contains stoichiometric amounts of actin and three actin related proteins (Arp4, Arp5, and Arp8). Arp4 and actin are essential for cell survival, but the viability of null mutants for Arp5 and Arp8 allowed us to further investigate their functional roles. We uncovered functions of Arp5 and Arp8 in chromatin remodeling by biochemical analysis of the null mutants. Moreover, the yeast SWR1 complex has been found to catalzye the exchange of the histone H2AZ variant. Given that the mechanism of incorporation of histone variants into chromatin has been obscure until now, our findings have dramatically changed perceptions of chromatin remodeling enzymes, and have provided an exciting new avenue for future investigations.