Nuclease‐Sensitive Regions on the Extrachromosomal r‐Chromatin from Tetrahymena pyriformis

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Abstract
The extrachromosomal DNA coding for the ribosomal RNA precursor in Tetrahymena contains a transcribed region with a size of 6 × 103 base pairs plus non‐transcribed central and distal spacers. In the present study the chromatin structure of the transcribed region and the terminal spacer have been compared. Micrococcal nuclease and DNase I were used to investigate the nucleosomal and the higher order structures. The specific DNA fragments were visualized by gel electrophoresis, Southern blotting onto nitrocellulose sheets and hybridization with specific 32P‐labelled RNA probes. Investigations of the cleavage patterns demonstrate the presence of a defined nucleosomal structure in the non‐transcribed region, while there is no indication of a nucleosomal pattern in the transcribed region. Specific regions on the r‐chromatin are hypersensitive to DNase I. The first cleavage occurs in the non‐transcribed central spacer region, while the second cleavage takes place in a region near the 3′ end. The hypersensitivity of the central part of r‐chromatin is also found by autodigestion in isolated nucleoli.