Nuclease-sensitive regions on the extrachromosomal r-chromatin from Tetrahymena pyriformis.

The extrachromosomal DNA coding for the ribosomal precursor in Tetrahymena contains a transcribed region with a size of 6 x 10(3) 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.