Visualization of chromatin in the yeast nucleus and nucleolus using hyperosmotic shock

Unlike in most eukaryotic cells, the genetic information of budding yeast in the exponential growth phase is only present in the form of decondensed chromatin, a configuration that does not allow its visualization in cell nuclei conventionally prepared for transmission electron microscopy. In this work, we have studied the distribution of chromatin and its relationships to the nucleolus using different cytochemical and immunocytological approaches applied to yeast cells subjected to hyperosmotic shock. Our results show that osmotic shock induces the formation of heterochromatin patches in the nucleoplasm and intranucleolar regions of the yeast nucleus. In the nucleolus, we further reveal the presence of osmotic shock-resistant DNA in the fibrillar cords which, in places, take on a pinnate appearance reminiscent of ribosomal genes in active transcription as observed after molecular spreading (‘Christmas trees’). We have also identified chromatin-associated granules whose size, composition, and behaviour after osmotic shock are reminiscent to that of mammalian perichromatin granules. Altogether, these data reveal it is possible to visualize heterochromatin in yeast and suggest the yeast nucleus displays a less-well compartmentalized organization than that of mammals.