| Summary: | The Polycomb group (PcG) are well known as epigenetic silencers of many target genes including important developmental regulators such as the Hox genes. PcG mutants also have mitotic phenotypes, though their cell cycle role is not understood. Depletion of the proximal isoform of the PcG protein Polyhomeotic (PhP) results in chromatin bridges during anaphase and telophase in embryos. PhP binds chromatin during S phase but not during mitosis in early embryos. As transcription rates of embryonic genes are low at this time, we suggest that PhP has a direct role in S phase. Time lapse imaging of php mutants reveals an acceleration of S phase timing, indicating that php regulates S phase length. Mutations in php do not affect DNA synthesis rates, but exhibit impaired ability to block cell cycle progression following exposure to x-rays, consistent with a role for PhP in the DNA damage response. PhP likely plays a direct role in the DNA damage checkpoint during S phase. PhP cofractionates and coimmunoprecipitates with the cohesin protein Pds5. Mutations in Pds5 suppress both the chromatin bridge phenotype and the S phase acceleration in php mutants. Other members of the core cohesin complex, with the exception of the cohesion establishment factor eco do not affect the chromatin bridge phenotype of php mutants. Despite this, the suppression of the S phase phenotypes of php is likely indirect, because Pds5 does not suppress the loss of the DNA damage response in php mutants. Instead, php is epistatic to Pds5 in the DNA damage response, which supports a role for php in the DNA damage checkpoint. The function of the complex containing Pds5 and PhP is likely related to silencing, as Pds5 strongly enhances the homeotic phenotypes of php, and Pds5 localizes to the Polycomb Group Response Elements (PREs) and promoter of the PcG target Ubx. This thesis demonstrates that PhP plays a role in the DNA damage checkpoint during S phase in addition to its role as an epigenetic silencer, and that Pds5 plays a role in silencing of Hox genes in addition to its role in the cell cycle. === Medicine, Faculty of === Medical Genetics, Department of === Graduate
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