Abstract
Synchronised cells of Tetrahymena pyriformis GL were labelled with 3H thymidine at a stage in the cell cycle when only the mitochondrial and extrachromosomal nucleolar ribosomal DNAs were replicating. In this way it was possible to prepare nuclei labelled selectively in the DNA of the ribosomal RNA genes. Since the ribosomal RNA cistrons of these cells are also very active in serving as a template for transcription, experiments were performed to test whether these genes are organised upon a nucleoprotein subunit structure of the kind that has been found in the total chromatin of a wide range of eukaryotic cell types. Tetrahymena macronuclei were prepared labelled uniformly in their DNA with 32P and labelled only in their nucleolar ribosomal DNA with 3H. Both the ribosomal genes and the bulk chromatin were then degraded in situ using micrococcal nuclease. The DNA fragments resulting from mild digestion were analysed on gels to reveal an identical DNA degradation pattern within both the ribosomal and bulk chromatins. It is concluded that the nucleoprotein structure of nucleolar rRNA cistrons posesses a periodic repeat along the DNA which is identical to that found in the substructure of unfractionated chromatin.