Variation in Chromatin Structure of the rDNA Transcribed Region in Human Peripheral Blood Lymphocytes

The rDNA transcribed region (TR) was tested for its accessibility for RsaI recognizing 15 TR sites, DNase I, and photoinducible arylazide (N-(4-azido-2-hydroxybenzoyl)-N,N"-diaminoheptane acetate) in isolated nuclei, and for the arylazide in intact cells. Arylazide readily entered the cells and did not appreciably affect the chromatin structure. Its photolysis products efficiently modified DNA in accessible sites. Single-strand breaks made by DNase I were not transformed into two-strand ones in rDNA TR, suggesting the necessity of denaturing electrophoresis for such an analysis. About 70% of all rDNA copies proved poorly accessible for endonucleases and arylazide, the accessibility being higher in their 18S and 5.8S rRNA gene regions than in the regions of the external transcribed spacers (ETSs) and the 28S rRNA gene. Proteinase K disrupted this structure, and the corresponding copies were extracted from nuclei. This explained whyin situ hybridization occasionally fails to reveal rDNA in the nucleolar fibrillar center (FC) on electron microscopic preparations. In other rDNA copies, TR (excluding 5"-ETS) was accessible for nucleases and arylazide. These copies were not extracted from nuclei treated with proteinase K. Some of their RsaI sites were protected by tightly bound proteins. Seven such regions were identified in TR. Possible association of the molecular structure, nucleolar location, and functional state of rDNA is discussed.