Histone H3 lysine 9 methylation has been proposed to provide a major "switch" for the functional organization of chromosomal subdomains. Here, we show that the murine Suv39h histone methyltransferases (HMTases) govern H3-K9 methylation at pericentric heterochromatin and induce a specialized histone methylation pattern that differs from the broad H3-K9 methylation present at other chromosomal regions. Suv39h-deficient mice display severely impaired viability and chromosomal instabilities that are associated with an increased tumor risk and perturbed chromosome interactions during male meiosis. These in vivo data assign a crucial role for pericentric H3-K9 methylation in protecting genome stability, and define the Suv39h HMTases as important epigenetic regulators for mammalian development.

Sister chromatid cohesion ensures the faithful segregation of chromosomes in mitosis and in both meiotic divisions. Meiosis-specific components of the cohesin complex, including the recently described SMC1 isoform SMC1 beta, were suggested to be required for meiotic sister chromatid cohesion and DNA recombination. Here we show that SMC1 beta-deficient mice of both sexes are sterile. Male meiosis is blocked in pachytene; female meiosis is highly error-prone but continues until metaphase II. Prophase axial elements (AEs) are markedly shortened, chromatin extends further from the AEs, chromosome synapsis is incomplete, and sister chromatid cohesion in chromosome arms and at centromeres is lost prematurely. In addition, crossover-associated recombination foci are absent or reduced, and meiosis-specific perinuclear telomere arrangements are impaired. Thus, SMC1 beta has a key role in meiotic cohesion, the assembly of AEs, synapsis, recombination, and chromosome movements.

Abstract. The preconditions and early steps of meiotic chromosome pairing were studied by fluorescence in situ hybridization (FISH) with chromosome-specific DNA probes to mouse and human testis tissue sections. Premeiotic pairing of homologous chromosomes was not detected in spermatogonia of the two species. FISH with centromere-and telomere-specific DNA probes in combination with immunostaining (IS) of synaptonereal complex (SC) proteins to testis sections of prepuberal mice at days 4-12 post partum was performed to study sequentially the meiotic pairing process. Movements of centromeres and then telomeres to the nuclear envelope, and of telomeres along the nuclear envelope leading to the formation of a chromosomal bouquet were detected during mouse prophase. At the bouquet stage, pairing of a mouse chromosome-8-specific probe was observed. SC-IS and simultaneous telomere FISH revealed that axial element proteins appear as large aggregates in mouse meiocytes when telomeres are attached to the nuclear envelope. Axial element formation initiates during tight telomere clustering and transverse filament-IS indicated the initiation of synapsis during this stage. Comparison of telomere and centromere distribution patterns of mouse and human meiocytes revealed movements of centromeres and then telomeres to the nuclear envelope and subsequent bouquet formation as conserved motifs of the pairing process. Chromosome painting in human spermatogonia revealed compacted, largely mutually exclusive chromosome territories. The territories developed into long, thin threads at the onset of meiotic prophase. Based on these results a unified model of the pairing process is proposed. p AIRING of homologous chromosomes during meiotic prophase of sexually reproducing organisms culminates in the formation of the synaptonemal complex (SC) 1 (for reviews see von Wettstein et al., 1984;Giroux, 1988). The SC is composed of axial elements (cores) that connect sister chromatids along their entire length. These become lateral elements when they get interconnected by transverse filaments to result in the well-known tripartite SC structure (Schmekel and Daneholt 1995). While chromosome pairing and meiotic recombination Address all correspondence to H. Scherthan,

Abstract. We have developed a novel technique for combined immunofluorescence/in situ hybridization on fixed budding yeast cells that maintains the threedimensional structure of the nucleus as monitored by focal sections of cells labeled with fluorescent probes and by staining with a nuclear pore antibody. Within the resolution of these immunodetection techniques, we show that proteins encoded by the SIR3, SIR4, and RAP1 genes colocalize in a statistically significant manner with Y' telomere-associated DNA sequences. In wild-type cells the Y' in situ hybridization signals can be resolved by light microscopy into fewer than ten loci per diploid nucleus. This suggests that telomeres are clustered in vegetatively growing cells, and that proteins essential for telomeric silencing are concentrated at their sites of action, i.e., at telomeres and/or subtelomeric regions. As observed for Rap1, the Sir4p staining is diffuse in a sir3-strain, and similarly, Sir3p staining is no longer punctate in a sir4-strain, although the derivatized Y' probe continues to label discrete sites in these strains. Nonetheless, the Y' FISH is altered in a qualitative manner in sir3 and sir4 mutant strains, consistent with the previously reported phenotypes of shortened telomeric repeats and loss of telomeric silencing.S EVERAL lines of evidence, including in situ hybridization with whole chromosome probes, suggest that the organization of chromosomes within the interphase nucleus is not random (for review see Cremer et al., 1993). Indeed, it is generally assumed that three-dimensional nuclear organization is likely to facilitate essential nuclear functions, such as transcription, the processing and transport of mRNA, replication, and recombination. Evidence for the organization of chromosomes in prophase nuclei was provided over a hundred years ago by Rabl's observation that salamander chromosomes are positioned in nuclei with centromeres clustered at one pole and the telomeres at the opposite pole (RAN, 1885). Work by Sedat and his colleagues later lent support to this notion through the study of the polytene salivary gland chromosomes of Drosophila melanogaster. They found that centromeres, fused into the chromocenter, abut the nuclear envelope within a restricted area while telomeres tended to cluster at the opposite pole (Mathog et al., 1984;Hochstrasser et al., 1986). A peripheral localization of telomeres has been also reported in Trypanosoma (Chung et

The 'chromosomal bouquet' is a polarized chromosomal arrangement that is highly conserved among eukaryotes. There have been many hypotheses about its role in the pairing of meiotic chromosomes, but until recently these have been difficult to test.