PcG recruitment by the YY1 REPO domain can be mediated by Yaf2

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.