Forum domain in Drosophila melanogaster cut locus possesses looped domains inside.

We have studied the relationship between chromosomal forum domains and looped domains in the cut locus of Drosophila melanogaster . Forum domains were earlier detected by separation in pulsed-field gels of 50-150 kb chromosomal DNA fragments obtained after spontaneous non-random degradation of chromosomes. We have localized the boundary region where cleavage sites are scattered between two forum domains in the regulatory region of the cut locus. We have sequenced a 13 kb region spanning few kilobases from distal domain, the boundary region and part of the proximal forum domain where several scaffold associated regions (SARs) were observed. We conclude that forum domains and looped domains are physically different types of domains and belong to different levels of organization in eukaryotic chromosomes. The boundary region between the neighboring forum domains in the cut locus possesses the Doc element insertion and a micro-satellite stretch and thus might remind a small island of heterochromatin and correspond to so-called intercalary heterochromatin that is known to be located in the 7B1-2 band where the major part of the cut locus is reside.