Drosophila Polycomb-group (PcG) proteins are transcriptional repressors that regulate gene expression throughout fly development. In larval wing imaginal discs, the hierarchical recruitment of three PcG complexes(PhoRC, which contains the DNA-binding proteins Pho and/or Pho1, followed by PRC2, which contains the E(z) chromatin-modifying subunit, and finally PRC1)to the bxd Polycomb Response Element (PRE) of Ultrabithorax(Ubx) represses the expression of this Hox gene. Now, on , Richard Jones and colleagues identify a new larval PcG complex, PCLC, which contains Polycomblike (Pcl), and reveal its place in this recruitment hierarchy. In embryos, Pcl associates mainly with PRC2, but in larvae, the researchers report, it is present in a larger complex that does not contain E(z). Other experiments place PCLC downstream of the binding of PhoRC to the bxdPRE, and upstream of PRC2 and PRC1 binding. The inclusion of Pcl in different PcG complexes at distinct developmental stages might indicate that different molecular activities are needed for gene silencing in fly embryos and larvae,suggest the researchers.
