Nucleosomes and other components of chromatin can act as potent regulators of transcription by modulating the access of regulatory proteins to DNA. By altering the structure and spacing of nucleosomes, members of the SWI2/SNF2 family of ATFases can activate or repress transcription. Much remains to be learned about how these chromatin-remodeling factors are targeted to specific genes; how their activities are regulated; and how they interact with each other and histone modifying enzymes to modulate chromatin structure and transcription. To address these important issues, we are studying three different chromatin-remodeling factors in Drosophila: Brahma (BRM), Kismet (KIS) and Imitation-switch (ISWI). Genetic studies have suggested that BRM and KIS play similar roles in transcriptional activation, while ISWI may act as a transcriptional repressor. BRM and ISWI function as the ATPase subunits of distinct chromatin remodeling complexes. The biochemical activities of KIS have not been characterized, but its sequence suggests that it also has chromatin remodeling activity. Our long-term goal is to elucidate the mechanism of action of KIS, ISWI and BRM. Genetic screens for enhancers and suppressors of phenotypes resulting from the expression of dominant-negative chromatin remodeling factors will be conducted to identify the genes and proteins with which they interact in vivo. DNA microarrays will be used to examine changes in gene expression caused by mutations in chromatin remodeling factors. Genetic assays will be used to investigate whether the acetylation of chromatin influences its ability to interact with chromatin remodeling factors. In a complementary series of biochemical experiments, KIS and associated proteins will be purified from embryo extracts and tested for the ability to remodel chromatin in vitro. Human homologs of Drosophila chromatin remodeling factors are involved in transcriptional regulation, viral integration, cell cycle control and cancer. BRG1, the human brm homolog, physically interacts with the retinoblastoma tumor suppressor protein; disruption of this interaction leads to malignant transformation. BRCA1 is a component of the BRG1 complex, suggesting that alterations in BRG1 function predispose individuals to breast and ovarian cancer. Furthermore, mutations in the INI 1 subunit of the BRM complex have been identified in malignant rhabdoid tumors. Thus, the information gained from the study of chromatin remodeling factors in Drosophila should be directly relevant to human disease.
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