CRYO-ELECTRON TOMOGRAPHIC STUDIES OF SACCHAROMYCES CEREVISIAE NUCLEAR ORGANIZATION

Abstract

In eukaryotes, DNA wraps around histone proteins to form nucleosomes. The current textbook model dictates that nucleosomes are packed into regular 30-nm fibers. The evidence for the fiber model comes mainly from in vitro experiments using reconstituted nucleosome arrays. However, more and more in situ studies on various samples, including human cell and plant, failed to find any high-order chromatin structures inside the nucleus. We proposed to study whether nucleosomes are packed into regular 30-nm chromatin fibers in S. cerevisiae. Yeast cells were synchronized, high-pressure frozen and cut into thin cryo-sections. By tomography of vitreous section, nucleosome densities were directly visualized in 3-D. Our results did not reveal any high-order chromatin structure. In the crowded nucleoplasm, nucleosome densities are distributed irregularly without any long-distance arrangement. Based on our and others' data, we propose that chromatin organization is likely regulated via local interactions among nucleosomes that are physically in proximity.