Chromosome pairing and recombination in S. pombe

Meiosis is a special nuclear division which in sexually reproducing organisms generates haploid cells that will ultimately form the progenitor cell for a unique individual organism or population of cells. This process is not only central to species continuation, but is one of the major driving forces in evolution. Understanding the molecular events which regulate meiosis is fundamental to our understanding of biology. Key events in meiosis are the pairing and reciprocal exchange of parts of homologous chromosomes (crossing over). Homologous chromosomes must locate one another, align and ultimately physically join by formation of recombining chromatids. In most eukaryotes a conserved and highly complex protein apparatus, the synaptonemal complex (SC), intimately links homologous chromosomes along their entire lengths and ensures the transformation of recombination precursor structures into crossovers and the subsequent reductional segregation of homologous chromosomes during the first meiotic division. The fission yeast, Schizosaccharomyces pombe, belongs to a group of very few eukaryotes that do not possess an SC. Instead, thin thread-like structures, the so-called linear elements (LEs) are present at the corresponding stages of meiosis. So far only a few proteins of LEs have been identified and they were recognized as being similar and even homologous to components of canonical SCs. Hence it was concluded that LEs are evolutionary relics of SCs. However, it is not clear which stage of LE development corresponds to which process at the chromatin or DNA level. It is a major goal of the proposed project to identify additional components of LEs and to establish a spatial and temporal relationship between LEs and meiotic recombination and chromosome pairing. By studying the parsimonious meiotic pairing apparatus of the fission yeast we hope to identify the minimal essential set of proteins and structural requirements for recombination site selection and the alignment of homologous chromosomes. This will help to further our understanding the processes of meiotic pairing and recombination.

Cells of sexually reproducing eukaryotes normally contain two equal (homologous) sets of chromosomes, one contributed by the father, the other by the mother during the fusion of gametes and the formation of a zygote. When eggs or sperm are produced, they must be furnished with a single set of chromosomes. Therefore, germ progenitor cells undergo a reductional division, meiosis. During meiosis, homologous chromosomes of paternal and maternal origin pair, exchange parts and segregate to different daughter nuclei. The meiotic pairing of homologous chromosomes is essential for their segregation to the gametes and hence for the endowment of the progeny with normal chromosome sets. Errors in this process lead to congenital defects in the progeny, stillbirths or miscarriages in humans. The aim of this project was the characterization of the so-called linear elements (LinEs) in the meiosis of the fission yeast, Schizosaccharomyces pombe. LinEs are formed instead of synaptonemal complexes which stabilize chromosome pairing in most other eukaryotes. LinEs are more simple structures and are believed to represent the minimal set of proteins required for chromosome pairing, and therefore, understanding their composition and function is of relevance to understanding chromosome pairing in all organisms. We identified novel proteinaceous components of LinEs by a yeast-two-hybrid screen with the LinE component Rec10 as the bait. One of them, the SUMO ligase Pli1, was characterized further. We could demonstrate that a yet unknown protein of the LinEs (possibly Rec10 itself) is SUMOylated, which causes a variety of posttranslational modifications of Rec10. Furthermore, by mass spectrometric analysis of a Rec10-containing complex, we identified Mug20 as another LinE component. Despite the termination of this project, studies on the function of Mug20 will be continued. In a collaboration with Jürg Kohli (University of Berne) we investigated yet another LinE component, Hop1. We showed that Hop1 is part of a mechanism to impede the sterically preferred recombination between sister chromatids and thereby promotes the required recombination of homologous chromosomes. Interestingly, the lack of most of these LinE components had little impact on the viability of gametes. Yet the LinE`s importance must be such that costly provisions such as their intricate morphological reorganization during meiosis, their complex composition, and their SUMO modification have been maintained throughout the evolution of S. pombe.