Budding yeast chromatin is dispersed in a crowded nucleoplasm in vivo

Chromatin organization has an important role in the regulation of eukaryotic systems. While recent studies have refined the 3-D models of chromatin organization with high resolution at the genome sequence level, little is known about how the most fundamental units of chromatin -- nucleosomes -- are positioned in 3-D in vivo. Here we have used electron cryotomography to study chromatin organization in the budding yeast S. cerevisiae. Direct visualization of yeast nuclear densities shows no evidence of 30-nm chromatin fibers. Aside from pre-ribosomes and spindle microtubules, few nuclear structures are larger than a tetranucleosome. Yeast chromatin does not form compact structures in interphase or mitosis and is consistent with being in an \"open\" configuration that is conducive to high levels of transcription. In the absence of higher-order chromatin packing, we propose that yeast can regulate its transcription using local nucleosome-nucleosome associations.