Yeast Silent Mating Type Loci Form Heterochromatic Clusters through Silencer Protein-Dependent Long-Range Interactions
Figure 9
Clustering of silent chromatin.
In the absence of silencing proteins, depicted as colored circles, HM-loci (HML- black rectangle; HMR- gray rectangle) are not silenced and they do not form clusters. Upon expression, silencing proteins are available throughout the nucleus and can become recruited to the HM silencers, independent of their sub-nuclear position. Heterochromatin is formed and silencing is established. Once these loci are silenced, they obtain the ability to form clusters that can also contain telomeres. Silent clusters create sub-nuclear compartments in which there is a high local concentration of silencing proteins available for recruitment to loci located within the cluster only. Membrane association of silenced loci will anchor clusters or unclustered loci to the nuclear periphery. Loci located outside heterochromatin compartments have no access to silencing proteins and are not silenced. In this model clustering, membrane anchoring as well a genomic proximity of HM loci to telomeres will all contribute to formation of sub-nuclear compartments that are enriched in silencing complexes, which will facilitate heterochromatin formation at resident loci.