The transcriptional activation of many yeast genes depends both on sequence-specific DNA binding proteins and on a recently identified large protein complex called the SWI/SNF complex. The yeast complex is thought to offset the repressive effects of chromatin, perhaps by facilitating the binding of DNA-binding proteins to their targets. Key components of the SWI/SNF complex are SW12/SNF2, an ATPase that may be a helicase, and SNF5, which is a novel protein. We have identified Drosophila homologs of SNF5 (snr1) and SW12 (brm) and have demonstrated the association of their products in a 2 x 10/6 dalton complex, about the same size as the yeast complex. The Drosophila genes are both required to activate homeotic (Hox) gene transcription, making them members of the trithorax (trx) group of activators. They oppose the actions of repression maintenance functions such as the Polycomb (Pc) group of genes. One mammalian SW12/SNF2 homolog has been reported to associate with Retinoblastoma protein, a mammalian homolog of brm inhibits growth of tumor cells in culture, and a SNF5 homolog has been reported to associate with HIV integrase. Trithorax itself encodes a protein with dramatic homology to HRX, a human leukemia proto-oncogene. We propose to use the power of Drosophila developmental genetics to learn the normal roles of snrl and brm which will shed light on their links to oncogenesis. A full genetic and developmental characterization of snr1 will be done to learn where and when the gene is required during development. Exact subnuclear locations of the proteins will be explored. To learn how these two proteins interact with other components of the brm/snr1 complex, and with proteins that are not stable components of the complex, we will use yeast two hybrid screening with either snr1 or brm as """"""""bait"""""""". To explore the molecular mechanism of brm/snr1 complex transcriptional activation, we will tether the two known component proteins to specific target genes in vivo to test for activation and see which other genes are required for that activation. To study specific target gene regulation by brm/snr1 complex, we will test the response of existing homeotic and other potential target gene enhancers to brm and snr1 mutations. Together these experiments will greatly clarify the molecular and developmental functions of the newly discovered complex.
