RNA granules act as egg timers

Determining the replication factor y settings S aner et al. describe how neighboring DNA regions stochas-tically assemble into replica-tion factories in budding yeast. A replicon is a stretch of DNA duplicated from a single replication origin. In eukaryotes, multiple replicons assemble into sub-nuclear structures called replication factories, where the replicons are duplicated by DNA polymerases and other replication proteins. Replication factories help to coordinate effi cient DNA synthesis, but how replicons are organized into these structures is unclear. Saner et al. used live-cell imaging to follow the replica-tion of different replicons along a budding yeast chromosome. Surprisingly, whether the replicons duplicated in the same or different replication factories varied from cell to cell. Neighboring replicons were more likely to duplicate in the same factory than replicons spaced further apart. Once assembled into the same factory, however, replicons remained associated for several minutes , long enough, perhaps, for replication to be completed. Using super-resolution microscopy, Saner et al. found that many replication factories contain only a single replicon and that few factories contain more than four. Mathematical models of factory assembly supported the idea that neighboring replicons randomly associate to form factories of variable size and composition. Senior author Tomoyuki Tanaka is now interested in determining whether similar principles apply to the formation of replication factories in mammalian nuclei and to the assembly of other sub-nuclear structures involved in transcription and DNA repair. KASH5 helps meiotic chromosomes LINC up H orn et al. identify a protein that helps homologous chromosomes pair up in meiosis by connecting them to the microtu-bule cytoskeleton. Early in meiosis, chromosomes cluster together so that homologous chromosomes can fi nd each other and pair up to undergo recombina-tion. Clustering is controlled by LINC complexes, which span the nuclear envelope to couple chromosomes to the microtubule-based motor protein cyto-plasmic dynein. Dynein can therefore pull chromosomes toward the centrosome on one side of the nucleus. LINC complexes are formed by members of the SUN and KASH protein families. SUN1 is an inner nuclear membrane protein that, in most organisms, attaches to the telomeres of meiotic chromosomes. A member of the KASH family of outer nuclear membrane proteins links SUN1 to cytoplasmic dynein, but which KASH protein performs this function in mammals is unknown. Horn et al. focused on KASH5, a recently identifi ed KASH protein expressed in testes and ovaries. KASH5 colocalized with SUN1 at sites where telomeres attached …