[Chromatin structure of ribosomal genes of Drosophila melanogaster. Random location of nucleosomes in DNA and characteristics of organization of non-transcribed spacer].

Chromatin structure of ribosomal genes from nuclei of Drosophila melanogaster embryos was studied by using micrococcal nuclease cleavage. End-directed labelling with short cloned fragments of the ribosomal repeat was carried out. It shows, that the micrococcal nuclease prefers specific sites in naked DNA, and the pattern of DNA cleavage is essentially conserved when the nuclei are digested. Only minor differences are visible. Hence, there are no specific positions of nucleosomes on the ribosomal repeat. The DNA fragments from the nuclei treated with micrococcal nuclease were electrophoresed, transferred to DBM-paper and hybridized with different probes subcloned from the ribosomal repeat. The non-transcribed spacer and the region of the beginning of transcription are hydrolysed significantly faster than the coding region or inactive ribosomal insertion. The region of NTS and the beginning of transcription do not give normal nucleosomal fragment in the range of 145-185 bp; instead they produce a heterogeneous band 200-280 bp in length even after prolonged digestion. Dinucleosomal fragments are also slightly longer and more heterogeneous than in other parts of the ribosomal repeat. Higher oligomers are similar throughout the ribosomal repeat. We suggest that a hypothetical transcription factor interacts in a way with histones and protects unusual fragments of DNA from digestion.