Structure of the active nucleolar chromatin of Xenopus laevis oocytes.

Active nucleolar chromatin of Xenopus laevis oocytes was prepared for electron microscopy by a step gradient method, which separates the chromatin from proteins and other constituents which might unspecifically bind at low ionic strength. Between putative RNA polymerases and within the non-transcribed spacer region, the chromatin appears as smooth, thin filaments, indistinguishable from pure DNA adsorbed to the same specimen. These filaments are found under all conditions tested, even in the presence of 100 mM NaCl. On the other hand, bulk rat liver chromatin, which was co-prepared with the active nucleolar chromatin, shows nucleosomes containing fibers, which condense into supranucleosomal structures with increasing ionic strength. Since the appearance and the behaviour of active nucleolar chromatin at different ionic strength and pH resembles that of pure DNA, but not that of any known type of chromatin, it is suggested that, except for the transcription apparatus, few macromolecular constituents are associated with ribosomal DNA during transcription.