It is not possible to clearly visualize how chromatin condenses to heterochromatin in vivo. However, in an in vitro system for Saccharomyces cerevisiae, the requirements for heterochromatin filament formation mirror those found in vivo. Here we report that the nucleosomes and the Sir2, Sir3 and Sir4 proteins, which are required for in vitro filament assembly, are also components of these filaments, confirming that the filaments are SIR-nucleosome filaments. We show the individual localization patterns of the Sir proteins on this SIR-nucleosome filament, and demonstrate that the metabolite, AAR, plays a specific and essential role in promoting the formation of this SIR-nucleosome filament
