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Budding yeast Mms22 is required for homologous recombination (HR)-mediated repair of stalled or broken DNA replication forks. Here we identify a human Mms22-like protein (MMS22L) and an MMS22L-interacting protein, NFκBIL2/TONSL. Depletion of MMS22L or TONSL from human cells causes a high level of double-strand breaks (DSBs) during DNA replication. Both proteins accumulate at stressed replication forks, and depletion of MMS22L or TONSL from cells causes hypersensitivity to agents that cause S phase-associated DSBs, such as topoisomerase (TOP) inhibitors. In this light, MMS22L and TONSL are required for the HR-mediated repair of replication fork-associated DSBs. In cells depleted of either protein, DSBs induced by the TOP1 inhibitor camptothecin are resected normally, but the loading of the RAD51 recombinase is defective. Therefore, MMS22L and TONSL are required for the maintenance of genome stability when unscheduled DSBs occur in the vicinity of DNA replication forks.

Production of healthy gametes requires a reductional meiosis I division in which replicated sister chromatids co-migrate, rather than separating as in mitosis or meiosis II. Fusion of sister kinetochores during meiosis I may underlie sister chromatid co-migration in diverse organisms, but direct evidence for such fusion has been lacking. Here we studied native kinetochore particles isolated from yeast using laser trapping and quantitative fluorescence microscopy. Meiosis I kinetochores formed stronger attachments and carried more microtubule-binding elements than kinetochores isolated from cells in mitosis or meiosis II. The meiosis I-specific monopolin complex was both necessary and sufficient to drive these modifications. Thus, kinetochore fusion directs sister chromatid co-migration, a conserved feature of meiosis that is fundamental to Mendelian inheritance.

During eukaryotic cell division, chromosomes must be precisely partitioned to daughter cells. This relies on a mechanism to move chromosomes in defined directions within the parental cell. While sister chromatids are segregated from one another in mitosis and meiosis II, specific adaptations enable the segregation of homologous chromosomes during meiosis I to reduce ploidy for gamete production. Many of the factors that drive these directed chromosome movements are known, and their molecular mechanism has started to be uncovered. Here we review the mechanisms of eukaryotic chromosome segregation, with a particular emphasis on the modifications that ensure the segregation of homologous chromosomes during meiosis I.

No abstract

The monopolin complex is a multifunctional molecular crosslinker, which in S. pombe binds and organises mitotic kinetochores to prevent aberrant kinetochore-microtubule interactions. In the budding yeast S. cerevisiae, whose kinetochores bind a single microtubule, the monopolin complex crosslinks and mono-orients sister kinetochores in meiosis I, enabling the biorientation and segregation of homologs. Here, we show that both the monopolin complex subunit Csm1 and its binding site on the kinetochore protein Dsn1 are broadly distributed throughout eukaryotes, suggesting a conserved role in kinetochore organisation and function. We find that budding yeast Csm1 binds two conserved motifs in Dsn1, one (termed Box 1) representing the ancestral, widely conserved monopolin binding motif and a second (termed Box 2-3) with a likely role in enforcing specificity of sister kinetochore crosslinking. We find that Box 1 and Box 2-3 bind the same conserved hydrophobic cavity on Csm1, suggesting competition or handoff between these motifs. Using structure-based mutants, we also find that both Box 1 and Box 2-3 are critical for monopolin function in meiosis. We identify two conserved serine residues in Box 2-3 that are phosphorylated in meiosis and whose mutation to aspartate stabilises Csm1-Dsn1 binding, suggesting that regulated phosphorylation of these residues may play a role in sister kinetochore crosslinking specificity. Overall, our results reveal the monopolin complex as a broadly conserved kinetochore organiser in eukaryotes, which budding yeast have co-opted to mediate sister kinetochore crosslinking through the addition of a second, regulatable monopolin binding interface.Electronic supplementary materialThe online version of this article (10.1007/s00412-019-00700-0) contains supplementary material, which is available to authorized users.

In the Neisseria spp., natural competence for transformation and homologous recombination generate antigenic variants through creation of mosaic genes (such as opas) and through recombination with silent cassettes (such as pilE/pilS) and gene-complement diversity through the horizontal exchange of whole genes or groups of genes, in minimal mobile elements (MMEs). An MME is a region encompassing 2 conserved genes between which different whole-gene cassettes are found in different strains, which are chromosomally incorporated solely through the action of homologous recombination. Comparative analyses of the neisserial genome sequences identified 39 potential MME sites, the contents of which were investigated in 11 neisserial strains. One hundred and eight different MME regions were identified, 20 of which contain novel sequences and these contain 12 newly identified neisserial coding sequences. Neisserial uptake signal sequences are associated with 38 of the 40 MMEs studied. In some sites, divergent dinucleotide signatures of the sequences between the flanking genes suggest relatively recent horizontal acquisition of some cassettes. The neisserial MMEs were used to interrogate all of the other available bacterial genome sequences, revealing frequent conservation of the flanking genes combined with the presence of different gene cassettes between them. In some cases, these sites can definitively be classified as MMEs in these other genera. These findings provide additional evidence for the MME model, indicate that MME-directed investigations are a good basis for the identification of novel strain-specific genes and differences within bacterial populations and demonstrate that these elements are probably ubiquitously involved in genetic exchange, particularly in naturally competent bacteria.

Highlights d The composition of meiotic centromeres and kinetochores is revealed d Kinetochores undergo extensive changes between meiotic prophase I and metaphase I d The Ctf19 CCAN orchestrates meiotic kinetochore specialization d A Ctf19 CCAN-directed kinetochore assembly pathway is uniquely critical in meiosis