ABSTRACT
Long-range chromosome organization is known to influence nuclear function. Budding yeast centromeres cluster near the spindle pole body, whereas telomeres are grouped in five to eight perinuclear foci. Using live microscopy, we examine the relative positions of right and left telomeres of several yeast chromosomes. Integrated lac and tet operator arrays are visualized by their respective repressor fused to CFP and YFP in interphase yeast cells. The two ends of chromosomes 3 and 6 interact significantly but transiently, forming whole chromosome loops. For chromosomes 5 and 14, end-to-end interaction is less frequent, yet telomeres are closer to each other than to the centromere, suggesting that yeast chromosomes fold in a Rabl-like conformation. Disruption of telomere anchoring by deletions of YKU70 or SIR4 significantly compromises contact between two linked telomeres. These mutations do not, however, eliminate coordinated movement of telomere (Tel) 6R and Tel6L, which we propose stems from the territorial organization of yeast chromosomes.
Subject(s)
Chromosomes, Fungal/physiology , Saccharomyces cerevisiae/physiology , Telomere/physiology , Cell Nucleolus/physiology , Cell Nucleus/physiology , DNA-Binding Proteins/genetics , Fluorescence Recovery After Photobleaching , G1 Phase/physiology , Gene Deletion , Genotype , Interphase/physiology , Luminescent Proteins/genetics , Microscopy, Fluorescence , Recombinant Fusion Proteins/genetics , Repressor Proteins/genetics , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae Proteins/genetics , Silent Information Regulator Proteins, Saccharomyces cerevisiae/genetics , Spindle Apparatus/physiologyABSTRACT
Epigenetic mechanisms silence the HM mating-type loci in budding yeast. These loci are tightly linked to telomeres, which are also repressed and held together in clusters at the nuclear periphery, much like mammalian heterochromatin. Yeast telomere anchoring can occur in the absence of silent chromatin through the DNA end binding factor Ku. Here we examine whether silent chromatin binds the nuclear periphery independently of telomeres and whether silencing persists in the absence of anchorage. HMR was excised from the chromosome by inducible site-specific recombination and tracked by real-time fluorescence microscopy. Silent rings associate with the nuclear envelope, while nonsilent rings move freely throughout the nucleus. Silent chromatin anchorage requires the action of either Ku or Esc1. In the absence of both proteins, rings move throughout the nucleoplasm yet remain silent. Thus, transcriptional repression can be sustained without perinuclear anchoring.