SUUR joins separate subsets of PcG, HP1 and B-type lamin targets in Drosophila.

Drosophila melanogaster Suppressor of Under-Replication (SuUR) gene encodes a protein that modulates replicative properties of heterochromatin in endocycles of polytene cells. The SuUR mutation abolishes underreplication of intercalary heterochromatin and results in partial underreplication of pericentric heterochromatin. We performed a genome-wide mapping of SUUR target genes in non-polytenic Drosophila Kc cells by using the DamID approach. We show that SUUR preferentially binds genes that are transcriptionally silent and late-replicated. Distinct subsets of SUUR targets are associated with PcG proteins (Pc and Esc; Polycomb and Extra sexcombs), heterochromatic proteins [HP1 and SU(VAR)3-9] and B-type lamin. The SUUR binding profile negatively correlates with the DNA polytenization levels of salivary gland polytene chromosomes. Finally, SUUR target genes are repressed in Drosophila embryos and gradually activated later in development. Together these results suggest that SUUR is a ubiquitous marker of heterochromatin in different cell types.