Chromatin structure of the telomeric region and 3'-nontranscribed spacer of Tetrahymena ribosomal RNA genes.

The chromatin structure of the 3'-nontranscribed spacer of the linear rRNA gene molecules of Tetrahymena thermophila was examined. This region includes the transcription termination site, two sets of recently identified conserved spacer repeats (Type IV and V repeats (Challoner, P. B., Amin, A. A., Pearlman, R. E., and Blackburn, E. H. (1985) Nucleic Acids Res. 13, 2661-2680], and the terminus of the molecule. Using sensitivity to nucleases as a probe, a unique chromatin structure was found in this rDNA region. Proceeding from the end of the rDNA molecule, the telomeric repeated sequence, (CCCCAA)n, was packaged in a non-nucleosomal complex adjacent to three phased nucleosomes. This nucleosomal structure was disrupted at the Type V repeat region, which, compared with the neighboring nucleosomal region, was more accessible to nucleases and, from both micrococcal nuclease and DNase I digestion results, was packaged in chromatin differently from the sequences flanking it on both sides. The region between the Type V repeats and adjacent to the transcription termination site was in yet another distinguishable chromatin structure as judged by its sensitivity to nucleases. It includes sites protected in chromatin and sites which were cleaved in chromatin but not detectably digested in DNA controls, suggesting that specific proteins are also associated with this region.