Ty Element Retrotransposition in Saccharomyces cerevisiae

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Over the past year, we have made progress in the following areas. Despite their evolutionary distance, the <I>Saccharomyces cerevisiae</i> retrotransposon Ty1 and retroviruses use similar strategies for replication, integration, and interactions with their hosts. We have examined the formation of circular Ty1 DNA, which is comparable to the dead-end circular products that arise during retroviral infection. Appreciable levels of circular Ty1 DNA are present with one-long terminal repeat (LTR) circles and deleted circles comprising major classes, while two-LTR circles are enriched when integration is defective. One-LTR circles persist when homologous recombination pathways are blocked by mutation, suggesting that they result from reverse transcription. Ty1 autointegration events readily occur, and many are coincident with and dependent upon DNA flap structures that result from DNA synthesis initiated at the central polypurine tract. These results suggest that Ty1-specific mechanisms minimize copy number and raise the possibility that special DNA structures are a targeting determinant. In a collaboration with Jonathan Keller (SAIC, Inc), we have studied the function of p205, which is a member of the interferon-inducible p200 family of proteins that regulate cell proliferation. Over-expression of p205 inhibits cell growth, although its mechanism of action is currently unknown. Therefore, we have evaluated the effect of p205 on the p53 and Rb-dependent pathways of cell cycle regulation. p205 expression results in elevated levels of p21, and activates the p21 promoter <I>in vitro</i> in a p53-dependent manner. In addition, p205 induces increased expression of Rb, and binds directly to Rb and p53. Interestingly, p205 also induces growth inhibition independent of p53 and Rb by delaying G2/M progression in proliferating cells, and is a substrate for Cdk2 kinase activity. Finally, we have identified other binding partners of p205 by a yeast two-hybrid screen, including the paired homeodomain protein HoxB2. Taken together, our results indicate that p205 induces growth arrest by interaction with multiple transcription factors that regulate the cell cycle, including but not entirely dependent on the Rb- and p53-mediated pathways of growth inhibition.