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

Abstract

The extrachromosomal DNA coding for the rRNA precursor in Tetrahymena contains a transcribed region with a size of 6 .times. 103 base pairs plus nontranscribed central and distal spacers. The chromatin structure of the transcribed region and the terminal spacer were 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-labeled RNA probes. Investigations of the cleavage patterns demonstrate the presence of a defined nucleosomal structure in the nontranscribed region, but there is no indication of a nucleosomal pattern in the transcribed region. Specific regions on the r[ribosomal]-chromatin are hypersensitive to DNase I. The 1st cleavage occurs in the nontranscribed central spacer region, while the 2nd cleavage takes place in a region near the 3'' end. The hypersensitivity of the central part of r-chromatin is found by autodigestion in isolated nucleoli.