The histone content of Tetrahymena ribosomal gene-containing chromatin

The histone composition of the ribosomal gene containing chromatin (rChromatin) of the ciliated protozoan Tetrahymena pyriformis has been investigated using purified nucleolar fractions in which ribosomal DNA constitutes 65-82% of the total DNA. In isolated nucleoli, rChromatin largely retains the periodic activated structure characteristic of rChromatin in the nucleus. For all five major histone classes, the histone to DNA ratios of nucleolar chromatin are similar to those of bulk macronuclear chromatin. These results argue that the differences between activated rChromatin and inactivated chromatin are not due to a deficiency in the number of histones available to form nucleosomes on the rDNA.