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* Department of Molecular Biology, Max Planck Institute for Biophysical Chemistry, 370170 Göttingen, Germany; and The Drosophila protein Hrb57A has sequence homology to mammalian heterogenous nuclear
ribonucleoprotein (hnRNP) K proteins. Its in vivo distribution has been studied at high resolution by confocal laser scanning microscopy (CLSM) in embryos injected with fluorescently labeled monoclonal antibody.
Injection of antibody into living embryos had no apparent deleterious effects on further development. Furthermore, the antibody-protein complex could be observed for more than 7 cell cycles in vivo, revealing a
dynamic redistribution from the nucleus to cytoplasm
at each mitosis from blastoderm until hatching. The
evaluation of two- and three-dimensional CLSM data
sets demonstrated important differences in the localization of the protein in the nuclei of living compared to
fixed embryos. The Hrb57A protein was recruited to
the 93D locus upon heat shock and thus serves as an in
vivo probe for the activity of the gene in diploid cells of
the embryo. Observations during heat shock revealed
considerable mobility within interphase nuclei of this transcription site. Furthermore, the reinitiation as well
as the down regulation of transcriptional loci in vivo
during the recovery from heat shock could be followed
by the rapid redistribution of the hnRNP K during
stress recovery. These data are incompatible with a
model of the interphase nucleus in which transcription complexes are associated with a rigid nuclear matrix.
Institute
of Biology, Department of Cytogenetics, Humboldt University of Berlin, 10115 Berlin, Germany
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