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A Polycomb group protein complex with sequence-specific DNA-binding and selective methyl-lysine-binding activities.

Polycomb response elements (PREs) are specific cis-regulatory sequences needed for transcriptional repression of HOX and other target genes by Polycomb group (PcG) proteins. Among the many PcG proteins known in Drosophila, Pho is the only sequence-specific DNA-binding protein. To gain insight into the function of Pho, we purified Pho protein complexes from Drosophila embryos and found that Pho exists in two distinct protein assemblies: a Pho-dINO80 complex containing the Drosophila INO80 nucleosome-remodeling complex, and a Pho-repressive complex (PhoRC) containing the uncharacterized gene product dSfmbt. Analysis of PhoRC reveals that dSfmbt is a novel PcG protein that is essential for HOX gene repression in Drosophila. PhoRC is bound at HOX gene PREs in vivo, and this targeting strictly depends on Pho-binding sites. Characterization of dSfmbt protein shows that its MBT repeats have unique discriminatory binding activity for methylated lysine residues in histones H3 and H4; the MBT repeats bind mono- and di-methylated H3-K9 and H4-K20 but fail to interact with these residues if they are unmodified or tri-methylated. Our results establish PhoRC as a novel Drosophila PcG protein complex that combines DNA-targeting activity (Pho) with a unique modified histone-binding activity (dSfmbt). We propose that PRE-tethered PhoRC selectively interacts with methylated histones in the chromatin flanking PREs to maintain a Polycomb-repressed chromatin state.

Pubmed ID: 16618800


  • Klymenko T
  • Papp B
  • Fischle W
  • Köcher T
  • Schelder M
  • Fritsch C
  • Wild B
  • Wilm M
  • Müller J


Genes & development

Publication Data

May 1, 2006

Associated Grants


Mesh Terms

  • Amino Acid Sequence
  • Animals
  • Animals, Genetically Modified
  • Chromatin
  • DNA-Binding Proteins
  • Drosophila Proteins
  • Drosophila melanogaster
  • Embryo, Nonmammalian
  • Gene Silencing
  • Histones
  • Homeodomain Proteins
  • Lysine
  • Methylation
  • Molecular Sequence Data
  • Polycomb-Group Proteins
  • Protein Binding
  • Transcription Factors