Abstract
The chromatin structure of a single cluster of six tandemly repeated 5S ribosomal RNA genes (5S genes) in Tetrahymena thermophila has been characterized. Indirect end labeling experiments indicate that the actively transcribed 5S genes in macronuclei are rapidly cut by DNAse I near the putative internal promotor and just 5' to the transcribed region. When cells are starved to reduce 5S gene transcription rates, the DNAse I sensitivity of the intragenic site is reduced relative to the 5' site. In the nontranscribed 5S genes in micronuclei, neither of these sites is hypersensitive to DNAse I. Thus structural alterations accompany both the activation of transcription during macronuclear development and physiological changes in the rate of transcription of the 5S genes. These DNAse I data together with studies using Staphylococcal nuclease suggest that rapidly transcribed 5S genes may not be associated with histones as nucleosomes. In contrast, the genes in starved cell macronuclei appear to be associated with one nucleosome per 280 base pair tandem repeat.