Sumo Modification Impedes Transcriptional Silencing by the Polycomb Group Repressor Sex Comb on Midleg

The Drosophila protein Sex Comb on Midleg (Scm) is a member of the Polycomb group (PcG), a set of transcriptional repressors that maintain silencing of homeotic genes during development. Recent findings have identified PcG proteins both as targets for modification by the ubiquitin-like protein SUMO and as catalytic components of the SUMO conjugation pathway. We have found that the SUMO conjugating enzyme Ubc9 binds to Scm and that this interaction, which requires the Scm C-terminal Sterile alpha motif (SAM) domain, is crucial for the efficient sumoylation of Scm. Scm is associated with the major Polycomb response element (PRE) of the homeotic gene Ultrabithorax (Ubx) and efficient PRE recruitment requires an intact Scm SAM domain. Global reduction of sumoylation augments binding of Scm to the PRE. This is likely to be a direct effect of Scm sumoylation since mutations in the SUMO acceptor sites in Scm enhance its recruitment to the PRE, while translational fusion of SUMO to the Scm N-terminus interferes with this recruitment. In the metathorax, Ubx expression promotes haltere formation and suppresses wing development. When SUMO levels are reduced we observe decreased expression of Ubx and partial haltere-to-wing transformation phenotypes. These observations suggest that SUMO negatively regulates Scm function by impeding its recruitment to the Ubx major PRE.