The Polycomb Group (PcG) complex of transcriptional repressors is critical for the maintenance of stage-specific developmental gene expression, stem cell maintenance and for large-scale chromosomal dynamics. Functional deficiency of a single PcG gene can severely compromise PcG function, leading to developmental defects, embryonic lethality, or a number of malignancies. Despite the critical nature of PcG proteins, the mechanisms by which these complexes mediate their effects are relatively uncharacterized. Nearly all vertebrate PcG proteins lack inherent DNA binding capacity, making it unclear how they are targeted to Polycomb response element (PRE) sequences. Transcription factor YY1 is a functional ortholog of a Drosophila PcG protein, Pleiohomeotic (PHO), one of the few PcG proteins with specific DNA binding capability, and YY1 can recruit PcG proteins to specific DNA sequences. A small 25 amino acid YY1 domain (the REPO domain) is necessary and sufficient for recruitment of PcG proteins to DNA and for transcriptional repression. We show here that the YY1 REPO domain interacts with PcG protein Yaf2 and recruits Yaf2 to DNA. Interaction is lost when the YY1 REPO domain is deleted. In addition we show that Yaf2, when linked to a heterologous DNA binding domain, can recruit PcG proteins to DNA leading to transcriptional repression. When the Drosophila homolog of Yaf2 (dRYBP) is mutated, PcG recruitment to DNA is reduced. Taken together, our results suggest that Yaf2 serves as a molecular bridge between YY1 and other PcG complex proteins.

The Yin Yang 1 (YY1) REPO domain was found to be necessary and sufficient to recruit Polycomb proteins to DNA and silence a transgeneic reporter gene (PNAS 2006 103:19296‐19301). The objective in this study was to characterize the molecular mechanisms responsible for Polycomb recruitment by REPO. Yin Yang 1 Associated Factor 2 (Yaf2) was identified as a putative ligand for the REPO domain in a yeast two‐hybrid screen of a mouse 17 day embryonic cDNA library. Full length YY1 (1‐414) and the YY1 REPO domain (YY1 201‐226), but not the YY1 REPO deletion (YY1 1‐414del201‐226), were capable of interacting with Yaf2 in the two hybrid assay. These observations were validated in vivo using a chromatin immunoprecipitation assay. A transgenic Drosophila strain was prepared expressing flag‐tagged Yaf2 under control of the heat shock 70 promoter. This strain was crossed to a strain containing the transgenic reporter element (BGUZ) and either heat shock 70 driven‐Gal4 DNA binding domain (GALDBD) or the GALDBD fused to YY1 201‐226 (GALREPO). FlagYaf2 was immunoprecipitated at the BGUZ locus in the presence of GALREPO but not in the presence of GALDBD. Together, these results indicate that the YY1 REPO recruits Yaf2 to DNA and likely contributes to Polycomb‐mediated silencing. This work was funded by a faculty development grant awarded to F.W from Philadelphia University.