Live imaging of telomeres: yKu and Sir proteins define redundant telomere-anchoring pathways in yeast

Background: The positioning of chromosomal domains within interphase nuclei is thought to facilitate transcriptional repression in yeast. Although this is particularly well characterized for telomeres, the molecular basis of their specific subnuclear organization is poorly understood. The use of live fluorescence imaging overcomes limitations of in situ staining on fixed cells and permits the analysis of chromatin dynamics in relation to stages of the cell cycle. Results: We have characterized the dynamics of yeast telomeres and their associated domains of silent chromatin by using rapid time-lapse microscopy. In interphase, native telomeres are highly dynamic but remain within a restricted volume adjacent to the nuclear envelope. This constraint is lost during mitosis. A quantitative analysis of selected mutants shows that the yKu complex is necessary for anchoring some telomeres at the nuclear envelope (NE), whereas the myosin-like proteins Mlp1 and MIp2 are not. We are able to correlate increased telomeric repression with increased anchoring and show that silent chromatin is tethered to the NE in a Sir-dependent manner in the absence of the yKu complex. Sir-mediated anchoring is S phase specific, while the yKu-mediated pathway functions throughout interphase. Subtelomeric elements of yeast telomere structure influence the relative importance of the yKu-and Sir-dependent mechanisms. Conclusions: Interphase positioning of telomeres can be achieved through two partially redundant mechanisms. One requires the heterodimeric yKu complex, but not MIp1 and MIp2. The second requires Silent information regulators, correlates with transcriptional repression, and is specific to S phase.