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Chromosomes are the biologically relevant form of organization of the genome in all known forms of life. Eukaryotes, but not prokaryotes, possess basic proteins, which organize DNA as chromatin, possibly to suppress transcriptional noise and chemical instability and to foster phase separation. Eukaryotes also developed sustainable sexual reproduction, involving regular fusion and fission of genomes, the latter known as meiosis, which converts a diploid precursor cell into four haploid products. In meiosis, homologs from each parent must be identified, linked by chiasmata and finally be transported to opposite meiotic products. We currently address the mechanism of identification, initiated by Spo11 mediated double-strand break formation, as well, as homology search and repair. In most organisms a subset of DSBs is turned into crossing over, the basis of the physical linkage between the homologues. Taken together, we study chromosome segregation mediated by meiotic recombination.
Using budding yeast as model organism, we first established chromosome preparations suitable for immunofluorescence (IF) of yeast chromosomes. Fig. 1 shows spreads representing 10 consecutive stages from premeiotic S-phase until the end of meiosis, 4 haploid nuclei. DNA (blue, DAPI) is decorated with red foci (Smc6 – mediator of genome stability and Holliday-junction resolution) and green lines (Zip1 – mediator of synapsis). For molecular resolution, we perform ChIP-Seq, which yields genome-wide maps of proteins on DNA. For Spo11, we have developed a novel technology, Protec-Seq, which returns meiotic DNA lesions with single nucleotide precision. With this technique, we discovered that dsDNA pieces are excised from chromosomes. Fig. 2 shows such mapped, excised DNA pieces in the form of colored half circles. Of course, we also use the strength of our model system, genetics, to perform forward and reverse genetic screens, and use the ease of genetic manipulations including CRISPR.
1987 Franz Klein received a PhD in yeast genetics with U. Wintersberger (I. Krebsforschung), and moved to D. Schweizer, I. Botanik.
1991 post doc with Breck Byers, U. of Washington.
2000 habilitation in Genetics following the discovery of meiotic cohesin Rec8 and its central role in meiosis.
2008 Coordinator of SFB “Chromosome Dynamics”
2011 full Univ Prof, 2015 Chair of Chromosome Biology.
Our discovery of COM1/Sae2/CtIP and the defect in DSB repair and synapsis in com1 mutants shows that fully formed, tripartite SC depends on DSB repair (TEM of silver stained yeast spreads). Subsequently orthologs in plants and animals with roles in meiosis and genome stability were discovered. We also found that MRE11 is essential for meiotic DSB repair (Nairz and Klein, 1997; Prinz at all, 1997).
In collaboration with Kim Nasmyth, we discovered scRec8 and demonstrated its fundamental functions. Importantly, the pattern of its localization led us to propose that deposition of cohesin in S-phase is followed by removal of cohesin on chromosome arms to trigger anaphase I (Fig.3 C, D) and removal of cohesin at kinetochores to trigger anaphase II (Fig.3 E, Klein et al., 1999). Now in textbooks.
High resolution mapping of the break machinery revealed its presence at the sites of cohesin, that is, at the chromosome axis (Fig. 4 blue,red symbol). Because DNA breaks occur away from the axis, these sequences must transiently interact with the break machinery for cleavage (Fig.4 red star, Panizza, 2011). DSB formation during axis tethering is likely conserved in all Eukaryotes.
The Smc5/6/Mms21 complex is related to cohesin. We demonstrated that Smc5/6 has 2 independent roles: to prevent and to resolve JMs (Joint Molecules, double Holliday Junctions). Fig.5 shows lethal accumulation of unresolved JMs during synchronous meiosis in smc6-ts (Xaver et al., 2013). Smc5/6 may detect branched DNA molecules to inform helicases (Blooms) or resolvases (Mus81) about their geometry.
We knew for 35 years that meiotic recombination is initiated by chromosome breaks, but only now we discovered that in this process often whole chromosome pieces are chopped out. We developed a novel method to map those fragments, which resulted in unprecedented, single nucleotide accuracy and allowed new key observations, such as to identify a DNA bending motive at preferred cleavage sites.
The mechanism could lead to enhanced chromosome evolution, as well as to pathogenic mutations, if repair fails.
Transcription dynamically patterns the meiotic chromosome-axis interface.
Sun, Xiaoji; Huang, Lingzhi; Markowitz, Tovah E; Blitzblau, Hannah G; Chen, Doris; Klein, Franz; Hochwagen, Andreas
Smc5/6-Mms21 prevents and eliminates inappropriate recombination intermediates in meiosis.
Martin Xaver, Lingzhi Huang, Doris Chen and Franz Klein
Spo11-accessory proteins link double-strand break sites to the chromosome axis in early meiotic recombination.
Panizza, Silvia; Mendoza, Marco A; Berlinger, Marc; Huang, Lingzhi; Nicolas, Alain; Shirahige, Katsuhiko; Klein, Franz
Franz Klein is part of the SFB "Meiosis"
Franz Klein is speaker of the Special Research Area (SFB) "Chromosome Dynamics - Unraveling the function of chromosomal domains" funded by the Austrian Science Fund FWF. SFB's are peer-reviewed, highly interactive research networks, established to foster long-term, interdisciplinary co-operation of local research groups working on the frontiers of their thematic areas.
The Group Klein participates in in the special Doctoral Program 'Chromosome Dynamics' reviewed and funded by the Austrian Research Fund FWF.