SPECIFIC INTERACTIONS OF CHROMATIN WITH THE NUCLEAR-ENVELOPE - POSITIONAL DETERMINATION WITHIN THE NUCLEUS IN DROSOPHILA-MELANOGASTER

Specific interactions of chromatin with the nuclear envelope (NE) in e arly embryos of Drosophila melanogaster have been mapped and analyzed. Using fluorescence in situ hybridization, the three-dimensional posit ions of 42 DNA probes, primarily to chromosome 2L, have been mapped in nuclei of intact Drosophila embryos, revealing five euchromatic and t wo heterochromatic regions associated with the NE. These results predi ct that there are approximately 15 NE contacts per chromosome arm, whi ch delimit large chromatin loops of approximately 1-2 Mb. These NE ass ociation sites do not strictly correlate with scaffold-attachment regi ons, heterochromatin, or binding sites of known chromatin proteins. Pa irs of neighboring probes surrounding one NE association site were use d to delimit the NE association site more precisely, suggesting that p eripheral localization of a large stretch of chromatin is likely to re sult from NE association at a single discrete site. These NE interacti ons are not established until after telophase, by which time the nucle ar envelope has reassembled around the chromosomes, and they are thus unlikely to be involved in binding of NE vesicles to chromosomes follo wing mitosis. Analysis of positions of these probes also reveals that the interphase nucleus is strongly polarized in a Rabl configuration w hich, together with specific targeting to the NE or to the nuclear int erior, results in each locus occupying a highly determined position wi thin the nucleus.