A possible skeletal substructure of the macronucleus of tetrahymena

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Abstract
On removal of chromatin from isolated macronuclei of T. pyriformis, residual structures are obtained, the organization of which faithfully reflects the distinctive architecture of the macronucleus. Macronuclei are isolated by a new procedure in which cells are lysed by immersion in citric acid and Triton X-100. This method is rapid and efficient and leaves the nuclear structures stripped of nuclear envelope and nucleoli. The remaining interconnected chromatin bodies are structurally differentiated into a dense outer shell and a fibrillar inner core. The fibrillar component is identified as chromatin because it is removed upon digestion with DNase and extraction with 2 M NaCl. The dense shell of the chromatin body is unaffected by the digestion procedure, which leaves a skeletal structure comprised of hollow spherical bodies. Analysis of the protein composition by SDS (sodium dodecyl sulfate) acrylamide gel electrophoresis before and after digestion with DNase and RNase and high-salt extraction shows that histones are diminished, whereas the nonhistone protein composition remains unchanged. DNase not only extracts chromatin but also protects the nonchromatin structure from the otherwise disruptive effects of high-salt extraction. The method used for isolating the nuclei also affects the structure remaining after the digestion procedure; the citric acid/Triton X-100 method enhances the stability of the interconnected spherical bodies. The method for isolating nuclei and the procedure by which chromatin is extracted are both major factors contributing to the detection of a possible nonchromatin nuclear skeleton.