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J. Cell Biol.,
Volume 141, Number 2, April 20, 1998 469-481
* Department of Molecular Biology, Max Planck Institute for Biophysical Chemistry, 37070 Göttingen, Germany; The subcellular three-dimensional distribution of three polycomb-group (PcG) proteins Department
of Zoology, University of British Columbia, Vancouver, British Columbia, Canada; and § ZMBH, University of Heidelberg, Im
Neuenheimer Feld 282, 69120 Heidelberg, Germany
polycomb, polyhomeotic and posterior sex combsin fixed
whole-mount Drosophila embryos was analyzed by
multicolor confocal fluorescence microscopy. All three
proteins are localized in complex patterns of 100 or
more loci throughout most of the interphase nuclear
volume. The rather narrow distribution of the protein
intensities in the vast majority of loci argues against a
PcG-mediated sequestration of repressed target genes by aggregation into subnuclear domains. In contrast to
the case for PEV repression (Csink, A.K., and S. Henikoff. 1996. Nature. 381:529-531), there is a lack of correlation between the occurrence of PcG proteins and
high concentrations of DNA, demonstrating that the silenced genes are not targeted to heterochromatic regions within the nucleus. There is a clear distinction between sites of transcription in the nucleus and sites of
PcG binding, supporting the assumption that most PcG
binding loci are sites of repressive complexes. Although
the PcG proteins maintain tissue-specific repression for
up to 14 cell generations, the proteins studied here visibly dissociate from the chromatin during mitosis, and disperse into the cytoplasm in a differential manner.
Quantitation of the fluorescence intensities in the
whole mount embryos demonstrate that the dissociated
proteins are present in the cytoplasm. We determined
that <2% of PH remains attached to late metaphase
and anaphase chromosomes. Each of the three proteins
that were studied has a different rate and extent of dissociation at prophase and reassociation at telophase.
These observations have important implications for
models of the mechanism and maintenance of PcG- mediated gene repression.
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