THE DYNAMIC NUCLEAR REDISTRIBUTION OF AN HNRNP K-HOMOLOGOUS PROTEIN DURING DROSOPHILA EMBRYO DEVELOPMENT AND HEAT-SHOCK - FLEXIBILITY OF TRANSCRIPTION SITES IN-VIVO

The Drosophila protein Hrb57A. has sequence homology to mammalian hete rogenous nuclear ribonucleoprotein (hnRNP) K proteins. Its in vivo dis tribution has been studied at high resolution by confocal laser scanni ng microscopy (CLSM) in embryos injected with fluorescently labeled mo noclonal antibody, Injection of antibody into living embryos had no ap parent deleterious effects on further development, Furthermore, the an tibody-protein complex could be observed for more than 7 cell cycles i n vivo, revealing a dynamic redistribution from the nucleus to cytopla sm at each mitosis from blastoderm until hatching, The evaluation of t wo- and three-dimensional CLSM data sets demonstrated important differ ences in the localization of the protein in the nuclei of living compa red to fixed embryos. The Hrb57A protein was recruited to the 93D locu s 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 he at shock could be followed by the rapid redistribution of the hnRNP K during stress recovery. These data are incompatible with a model of th e interphase nucleus in which transcription complexes are associated w ith a rigid nuclear matrix.