UNUSUAL PROPERTIES OF GENOMIC DNA-MOLECULES SPANNING THE EUCHROMATIC HETEROCHROMATIC JUNCTION OF A DROSOPHILA MINICHROMOSOME

While investigating the copy number of minichromosome Dp(1;f)1187 sequ ences in the polyploid chromosomes of ovarian nurse and follicle cells of Drosophila melanogaster we discovered that restriction fragments s panning the euchromatic - heterochromatic junction of the chromosome a nd extending into peri-centromeric sequences had the unusual property of being selectively resistant to transfer out of agarose gels during Southern blotting, leading to systematic reductions in Dp1187-specific hybridization signals. This property originated from the peri-centrom eric sequences contained on the junction fragments and was persistentl y associated with Dp 1187 DNA, despite attempts to ameliorate the effe ct by altering experimental protocols. Transfer inhibition was unlikel y to be caused by an inherent physical property of repetitive DNA sequ ences since, in contrast to genomic DNA, cloned restriction fragments spanning the euchromatic - heterochromatic junction and containing rep etitive sequences transferred normally. Finally, the degree of inhibit ion could be suppressed by the addition of a Y chromosome to the genot ype. On the basis of these observations and the fact that pericentrome ric regions of most eukaryotic chromosomes are associated with cytolog ically and genetically defined heterochromatin, we propose that peri-c entromeric sequences of Dp1187 that are incorporated into heterochroma tin in vivo retain some component of heterochromatic structure during DNA isolation, perhaps a tightly bound protein or DNA modification, wh ich subsequently causes the unorthodox properties observed in vitro.