Polycomb group (PcG) protein complexes repress gene expression during the development of higher eukaryotes by binding to Polycomb group response elements (PREs). Little is known about how PcG complexes are recruited to PREs but, on , Lesley Brown and Judith Kassis suggest that Spps (Sp1-like factor for Pairing Sensitive-silencing) might be involved in this process in Drosophila. All known Drosophila PREs contain binding sites for Sp1/KLF zinc-finger proteins. The researchers now report that the Sp1/KLF family member Spps binds to Ubx and engrailed PREs, and to polytene chromosomes in a binding pattern that closely matches that of the PcG protein Psc. Spps deletion suppresses `pairing-sensitive silencing', they report, a PRE-associated activity in which somatic-chromosome pairing increases PcG-mediated repression. Spps mutation also enhances the phenotype of pho mutants; the PcG protein Pho is involved in, but not sufficient for, PcG complex recruitment to PREs. Together, these results suggest that Spps works with, or in parallel to, Pho to recruit PcG complexes to PREs.
