Genomic rearrangements linked to aberrant recombination are associated with cancer and human genetic diseases. Such recombination has indirectly been linked to replication fork stalling. Using fission yeast, we have developed a genetic system to block replication forks at nonhistone/DNA complexes located at a specific euchromatic site. We demonstrate that stalled replication forks lead to elevated intrachromosomal and ectopic recombination promoting site-specific gross chromosomal rearrangements. We show that recombination is required to promote cell viability when forks are stalled, that recombination proteins associate with sites of fork stalling, and that recombination participates in deleterious site-specific chromosomal rearrangements. Thus, recombination is a "double-edged sword," preventing cell death when the replisome disassembles at the expense of genetic stability.

The detection of DNA damage activates DNA repair pathways and checkpoints to allow time for repair. Ultimately, these responses must be coordinated to ensure that cell cycle progression is halted until repair is completed. Several multiprotein complexes containing members of the structural maintenance of chromosomes family of proteins have been described, including the condensin and cohesin complexes, that are critical for chromosomal organization. Here we show that the Smc5/Smc6 (Smc5/6) complex is required for a coordinated response to DNA damage and normal chromosome integrity. Fission yeast cells lacking functional Smc6 initiate a normal checkpoint response to DNA damage, culminating in the phosphorylation and activation of the Chk1 protein kinase. Despite this, cells enter a lethal mitosis, presumably without completion of DNA repair. Another subunit of the complex, Nse1, is a conserved member of this complex and is also required for this response. We propose that the failure to maintain a checkpoint response stems from the lack of ongoing DNA repair or from defective chromosomal organization, which is the signal to maintain a checkpoint arrest. The Smc5/6 complex is fundamental to genome integrity and may function with the condensin and cohesin complexes in a coordinated manner.Cell cycle checkpoints are surveillance mechanisms that monitor the order and fidelity of cell cycle events. Among these are the DNA integrity checkpoints, which monitor DNA replication and DNA damage. Ongoing or stalled replication activates a checkpoint that prevents initiation of mitosis, ensuring the interdependence of the S phase and mitosis to maintain ploidy. There is considerable overlap between this checkpoint and that which monitors DNA damage during the S and G 2 phases of the cell cycle, at which time there is the added challenge of coordinating cell cycle progression with successful DNA repair (8).The biology of the G 2 DNA damage checkpoint in the fission yeast Schizosaccharomyces pombe has been extensively studied (40). This checkpoint arrests the cell cycle through maintenance of the inhibitory tyrosine-15 phosphorylation of Cdc2, which is achieved through Chk1-dependent signaling to the Cdc25 phosphatase and the Wee1 kinase that regulate Cdc2 (15,39,41,42). The earliest molecular marker of the G 2 DNA damage response is Rad3-dependent phosphorylation of its binding partner Rad26, the homolog of human ATR and ATRIP (9, 12). Phosphorylation of Chk1 following DNA damage is mediated by Rad3 and requires several other proteins encoded by the checkpoint rad genes, rad1, hus1, rad9, and rad17, together with the BRCT domain protein, Crb2 (29,43,56). These proteins, and the Rad3 and Rad26 homologs, have also been shown to independently localize to the sites of DNA damage in both Saccharomyces cerevisiae and human cells (23,35,58). Although Rad3/ATR activation is an early checkpoint response to DNA damage, the actual sensors of DNA lesions are unknown and may differ depending on the specific nature of the lesion (3, 59).Exper...

The structural maintenance of chromosome (SMC) proteins are key elements in controlling chromosome dynamics. In eukaryotic cells, three essential SMC complexes have been defined: cohesin, condensin, and the Smc5/6 complex. The latter is essential for DNA damage responses; in its absence both repair and checkpoint responses fail. In fission yeast, the UV-C and ionizing radiation (IR) sensitivity of a specific hypomorphic allele encoding the Smc6 subunit, rad18-74 (renamed smc6-74), is suppressed by mild overexpression of a six-BRCT-domain protein, Brc1. Deletion of brc1 does not result in a hypersensitivity to UV-C or IR, and thus the function of Brc1 relative to the Smc5/6 complex has remained unclear. Here we show that brc1D cells are hypersensitive to a range of radiomimetic drugs that share the feature of creating lesions that are an impediment to the completion of DNA replication. Through a genetic analysis of brc1D epistasis and by defining genes required for Brc1 to suppress smc6-74, we find that Brc1 functions to promote recombination through a novel postreplication repair pathway and the structure-specific nucleases Slx1 and Mus81. Activation of this pathway through overproduction of Brc1 bypasses a repair defect in smc6-74, reestablishing resolution of lesions by recombination.

Nature 430, 332-335 (2004) In this Letter, we concluded, primarily on the basis of energy-dispersive X-ray diffraction, that metallic glass could be formed in pure zirconium under high pressure and temperature conditions. However, careful observations using an angular-dispersive method and imaging-plate detector, together with X-ray-transparent anvils (H. Saitoh, T. Hattori, H. Kaneko, Y. Okajima and W. Utsumi, unpublished results), have revealed that our conclusion was in error. We are now convinced that the disappearance of diffraction lines of zirconium observed in our energy-dispersive experiments should be interpreted instead as rapid crystal growth at temperatures above that of the q-b phase transformation. Our original misinterpretation was partly due to our assumption that significant crystalline growth could not occur at temperatures of less than one-third of the melting point of zirconium. To a greater extent, it was a result of the limitations of the experimental techniques then available, which only allowed viewing of a very narrow window of Debye diffraction rings and therefore increased the probability of missing the Bragg spots. The state-of-the-art techniques used by Saitoh et al. at the Synchrotron Radiation Research Center, Japan Atomic Energy Institute, Hyogo, are superior for high-pressure research on non-crystalline materials. We thank our Japanese colleagues for their careful work and for inviting us to observe their high pressure and temperature synchrotron X-ray diffraction experiments at beamline BL14B1 of SPring-8. RETRACTION Nature 436, 829-832 (2005) This Letter described several notable physical properties for low-doped ruthenium copper oxides, from which conclusions concerning superconductivity in copper oxides were drawn. A key result was the observation of an unusual, negative, expansion of the lattice parameters and volume at temperatures below the Ru spin-ordering transition, T Ru , as shown in Figure 3. Unfortunately, we have discovered that a discrepancy in the algorithm for fitting the diffrac-tion data led to erroneous shifts in the cell parameters and resulted in the apparent negative expansion. We retract the claim of negative lattice expansion and the consequent inferences concerning the existence of two states in the pseudogap regime of the copper oxide electronic phase diagram. However, an anomalous expansion of the separation between copper oxide planes below T Ru , as shown in Figure 3b, is reproducible and has subsequently been observed in other ruthenium copper oxide samples. Our results concerning large magnetoresistances and successive Ru and Cu spin-ordering transitions are not affected, so Figures 1 and 2 and the Supplementary Figures remain valid. We thank S. Kimber and A. Williams for undertaking further calculations to determine the nature of the fitting problem. RETRACTION An earlier paper 1 by two of us, in which it was concluded that expression of a hairpin RNA homologous to ura4 RNA in Schizo-saccharomyces pombe results in the production of siRNAs that br...