[Gradient condensation of chromatin in ribosomal genes of Drosophila melanogaster].

The organization of chromatin in D. melanogaster ribosomal repeats with and without insertions was studied. We have shown earlier that upon digestion with micrococcal nuclease a "non-transcribed" intergenic spacer produces unusual chromatin particles containing DNA fragments 200-280 b.p. in length. These particles sediment like H1-containing nucleosomes, are stable only in the presence of polyamines, and are probably bound to some non-histone protein. The content of core histones and H1 in different regions of ribosomal genes has been studied by two-dimensional electrophoresis of chromatin particles and by "protein-image" hybridization. The content of histones and respectively the degree of chromatin condensation increase in the following order: the 1kb-long region surrounding the initiation site is practically free of histones less than the region of 240 b.p. repeats from the intergenic spacer, containing homologies with the ribosomal promotor less than coding region preceding the usual site of insertions less than coding region lying behind this site less than inactive type II ribosomal insertion. Therefore, the region of the beginning of transcription of most ribosomal genes is in an active conformation, even though at least 75% of the genes are repressed. Ribosomal insertions are in a compact, repressed form. We suggest that their inhibitory action on the transcription of corresponding genes at the molecular level is similar to the position effect of heterochromatic regions at the chromosomal level.