text: We propose to develop further assays to identify the loci at which RNA:DNA hybrids form in the genome. We also plan to gather information that will give us insights into the function of these loci. The foundation for this proposal is a reasonably substantial amount of preliminary data that show an antibody detecting RNA:DNA hybrids can be used to immunoprecipitate such loci in a human cell line, allowing sequencing to map these loci back to the genome. We find that these loci have characteristics consistent with those predicted by in vitro and other prior studies, with polypurine skewing, enrichment in rDNA and telomeres and loci like the mtDNA origin of replication. We have some early insights into the function of these loci from their physical association with genes that are completely silenced, and from mass spectroscopy analysis of proteins immunoprecipitated from chromatin. Our funding proposal is based on the further development of the genome-wide mapping assay, including some orthogonal approaches using other nucleases, affinity reagents and chemical mutagens. We propose to get much more detailed mass spectroscopy data from different cell types from human and mouse, and perform rigourous validation of the candidates identified. Finally, we will perform bioinformatic studies o the sequences at which we see the RNA:DNA hybrids forming, extending our current analyses which show intriguing patterns of purine:pyrimidine skewing, and correlate their formation with functional outcomes like gene expression and DNA replication timing. A lot of the regulatory mechanisms for the genome assume underlying double stranded DNA as the default, but if RNA:DNA hybrids are formed at a certain locus it would change our assumptions about the ability of that locus to bind transcription factors, undergo DNA methylation or organize as nucleosomes. At the very least we will be identifying a variable that has the potential to confound some of these assumptions. We hope to take the insights to a higher level, identifying innate properties of these loci that will allow us to add a layer of information about how the genome functions. If this exploratory project is successful, we will have a clear idea how to expand the study to a more comprehensive project in the future. PHS 398/2590 (Rev. 11/07) Continuation Format Page