Ultrastructure and template accessibility of modified chromatin in isolated macronuclei of Paramecium aurelia.

Macronuclei of Paramecium aurelia, isolated in 4% gum arabic, contain nearly exclusively heterochromatin which is unaccessible to DNA-dependent, bacterial RNA polymerase. Heterochromatin decondensation under tris-HCl treatment did not change its template accessibility, whereas selective removal of the basic proteins induced profound changes in ultrastructure of heterochromatin and increased its template activity. More intensive incorporation of H3-UMP was found after arginine rich histones removal. The relations between functional and morphological changes after various heterochromatin modification in macronuclei of Paramecium aurelia, a representative of lower eukariotes, are discussed and compared with those in higher eukariotes.