Variegated chromatin structures of mouse ribosomal RNA genes

We have employed a chromatin fractionation procedure on micrococcal nuclease-digested nuclei to examine the chromatin structure of mouse ribosomal RNA genes in two systems that differ by at least 14-fold in the level of ribosomal RNA transcription. In a cultured cell line enriched in transcriptionally active ribosomal chromatin, most ribosomal sequences are preferentially sensitive to digestion by micrococcal nuclease, reside in an insoluble chromatin fraction, and lack typical nucleosomal packaging; only minor amounts of ribosomal sequences are packaged into soluble, nucleosomal chromatin. By contrast, in adult liver, which is enriched in transcriptionally inactive ribosomal chromatin, the majority of ribosomal genes are packaged into soluble, nucleosomal chromatin. However, a significant fraction of liver ribosomal chromatin is insoluble and possesses a non-nucleosomal structure. Therefore, within a single cell population or tissue, mouse ribomal RNA genes are organized into both nucleosomal and non-nucleosomal chromatin structures. We suggest that these structures have functional significance.