The association of ribosomal RNA genes with histones as a function of their expression has been studied in Xenopus luevis erythrocytes, where the genes are silent, and tadpoles at stage 40, where these genes are actively transcribed. Isolated nuclei were either treated with formaldehyde or irradiated with an ultraviolet laser to cross-link proteins to DNA. The covalently linked protein-DNA complexes were purified by centrifugation through CsCl and immunoprecipitated with antibodies against H1, H2A and H4. DNA from the precipitated complexes was analysed for the presence of ribosomal DNA sequences by hybridization to specific probes. The actively transcribed ribosomal genes from X . luevis embryos are associated with H1, H2A and H4 as are the non-transcribed genes in the erythrocytes.In the light of the progress made in understanding the structure of the nucleosome as the basic unit of chromatin, surprisingly little is known about its behavior during transcription. It is not presently known whether the nucleosomes impede the movement of transcribing RNA polymerases along DNA; nucleosomes have been identified on both repressed and some active genes (for reviews see 11 -3). It is not even known whether the histones interfere at all with the transcriptional machinery. Whilst the possibility for transient unfolding of nucleosomes immediately engaged in transcription offers some explanation for the contradictory results concerning the nucleosomal organization of active genes, whether such an unfolding is accompanied by dissociation of histones or whether they remain bound to DNA is not clear.Extensive studies in this respect have been focused on the genes for ribosomal DNA. Electron-microscopic visualization of ribosomal genes have revealed non-beaded chromatin fibers within active transcriptional units [4-81, in some cases indistinguishable from naked DNA [6, 81. It has been argued that the nucleolar chromatin from X . luevis oocytes is associated with a few proteins other than those involved in transcription [6]. Other investigations, however, contradict such a conclusion. The presence of organized, histone-containing particles in transcribed ribosomal chromatin was implied mainly from biochemical studies [9 -151. Consistent with such a view is the evidence from yeast that the nucleosome loss does not have a gcneral effect on transcription by all three polymerases [16,17]. Furthermore, data have been presented for a different nucleosome structure of transcribed ribosomal DNA chromatin from Physurum [12-141; studies on the exposure of the sulfhydryl group of histone H3 in these nucleosomes raised the possibility of nucleosome unfolding during transcription Correspondence to I. G. Pashev, Institute of Molecular Biology, Bulgarian Academy of Sciences, BG-1113 Sofia, Bulgaria without dissociation of histones from DNA [13]. Recently, two groups addressed these questions by investigating the effect on transcription of a single-positioned nucleosome on short linear DNA [18,19]. They showed that the prokaryotic SP6 polymera...