In eukaryotic cells, double-strand breaks (DSBs) in DNA are generally repaired by the pathway of homologous recombination or by DNA nonhomologous end joining (NHEJ). Both pathways have been highly conserved throughout eukaryotic evolution, but no equivalent NHEJ system has been identified in prokaryotes. The NHEJ pathway requires a DNA end-binding component called Ku. We have identified bacterial Ku homologs and show that these proteins retain the biochemical characteristics of the eukaryotic Ku heterodimer. Furthermore, we show that bacterial Ku specifically recruits DNA ligase to DNA ends and stimulates DNA ligation. Loss of these proteins leads to hypersensitivity to ionizing radiation in Bacillus subtilis. These data provide evidence that many bacteria possess a DNA DSB repair apparatus that shares many features with the NHEJ system of eukarya and suggest that this DNA repair pathway arose before the prokaryotic and eukaryotic lineages diverged.

Mammalian cells repair DNA double-strand breaks (DSBs) through either homologous recombination or non-homologous end joining (NHEJ). V(D)J recombination, a cut-and-paste mechanism for generating diversity in antigen receptors, relies on NHEJ for repairing DSBs introduced by the Rag1-Rag2 protein complex. Animals lacking any of the seven known NHEJ factors are therefore immunodeficient. Nevertheless, DSB repair is not eliminated entirely in these animals: evidence of a third mechanism, 'alternative NHEJ', appears in the form of extremely rare V(D)J junctions and a higher rate of chromosomal translocations. The paucity of these V(D)J events has suggested that alternative NHEJ contributes little to a cell's overall repair capacity, being operative only (and inefficiently) when classical NHEJ fails. Here we find that removing certain portions of murine Rag proteins reveals robust alternative NHEJ activity in NHEJ-deficient cells and some alternative joining activity even in wild-type cells. We propose a two-tier model in which the Rag proteins collaborate with NHEJ factors to preserve genomic integrity during V(D)J recombination.

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The covalent rejoining of DNA ends at single-stranded or double-stranded DNA breaks is catalysed by DNA ligases. Ligases catalyse the formation of phosphodiester bonds at nick sites between 3P-hydroxyl and 5P-phosphate termini in double-stranded DNA [1]. DNA ligases are required for a number of important cellular processes including the replication of DNA, repair of damaged DNA, and for various recombination events within the cell. The ligase enzyme family can be divided into two broad classes: those requiring NAD for activity and those requiring ATP [1]. The eukaryotic, viral and archaeal enzymes all require ATP, whereas the NADdependent ligases have only been found in bacteria. Despite their requirement for di¡erent co-factors both bacterial DNA ligases and their eukaryotic counterparts utilise a conserved catalytic mechanism [1]. Three DNA ligases (I, III and IV) have been identi¢ed in mammalian cells and it has been well documented that these distinct ligases play speci¢c roles within the cell [2]. All eubacterial genomes encode an NAD -dependent ligase and, until recently, it was commonly believed that this enzyme was solely responsible for all DNA ligation reactions. However, it is now evident, from the sequences of many bacterial genomes, that some bacteria contain two or more DNA ligases, including ATP-dependent ligases. The presence of these additional ATP-dependent ligases suggests that prokaryotes, like eukaryotes, may employ speci¢c ligases for de¢ned roles in vivo, such as DNA repair and recombination.We recently identi¢ed putative homologues of the Ku70 and Ku80 DNA repair proteins in many bacterial genomes [2]. Ku functions as a heterodimer in vivo that binds to the ends of DNA and promotes e¤cient and accurate DNA endjoining repair. Mammalian cells de¢cient in Ku are sensitive to ionising radiation. In our previous report we noted that many, but not all, of the bacterial Ku homologues are organised into operons containing putative ATP-dependent DNA ligases [2]. These include ligases encoded by YkoU (Bacillus subtilis), BH2209 (Bacillus halodurans), Rv0938 (Mycobacterium tuberculosis), PA2150 (Pseudomonas aeruginosa), and AFI1725 (Archaeoglobus fulgidus) (for a full sequence alignment of ATP-dependent ligases see www.ncbi.nlm.nih.gov/cgibin/COG/palox?COG1793). To detect related DNA ligases in other bacterial genomes we used YkoU and its orthologues as query sequences in BLAST searches of the protein sequence database. These searches retrieved additional putative ATPdependent ligase orthologues (Mll2077, Mll4606, Mll9625M esorhizobium loti and SMB20685, SMA0424^Sinorhi-zobium loti) within operons containing Ku-like genes. We also noted the presence of previously undetected Ku-like genes in the genome of S. loti (SMb20686, SMb21406, SMb21407, SMa0426) and in many un¢nished bacterial genomes including Bacillus anthracis (http://www.tigr.org/ cgi-bin/BlastSearch/blast.cgi ?organism = b_anthracis).An examination of the secondary structure of the Ku-associated DNA ligases suggested that these putative p...

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Recent studies in Escherichia coli indicate that the interconversion of DNA replication fork and Holliday junction structures underpins chromosome duplication and helps secure faithful transmission of the genome from one generation to the next. It facilitates interplay between DNA replication, recombination and repair, and provides means to rescue replication forks stalled by lesions in or on the template DNA. Insight into how this interconversion may be catalysed has emerged from genetic, biochemical and structural studies of RecG protein, a member of superfamily 2 of DNA and RNA helicases. We describe how a single molecule of RecG might target a branched DNA structure and translocate a single duplex arm to drive branch migration of a Holliday junction, interconvert replication fork and Holliday junction structures and displace the invading strand from a D loop formed during recombination at a DNA end. We present genetic evidence suggesting how the latter activity may provide an efficient pathway for the repair of DNA double-strand breaks that avoids crossing over, thus facilitating chromosome segregation at cell division.

L'effet des rapports « metropolitan-hinterland » sur la vie politique des « hinterland » a été largement ignorée jusqu'ici. La plupart des analyses du régionalisme au Canada ne traitent pas des sous-régions provinciales. Cet article tente donc de combler partiellement cette lacune en analysant dans le nord-ouest ontarien les différents types de politiques et les facteurs qui les déterminent.Après avoir, analysé les principales caractéristiques de cette région, l'auteur soutient qu'il y a trois différents types de politique qu'il désigne ainsi : la politique de l'Extraction, la politique de la Frustration et l'esprit de clocher. La politique de l'Extraction se compose de deux processus. D'abord, il y a la politique de la futilité constituée par les tentatives faites par les autochtones pour obtenir des changements socioéconomiques majeurs, ou, à tout le moins, le plus de concessions possibles de la métropole. Ensuite, il y a la politique de « gratification » qui consiste pour la métropole à essayer d'extraire les ressources locales au coût le plus bas possible sans avoir à modifier de manière fondamentale la nature de la relation. La politique de la Frustration, tout comme l'esprit de clocher, sont présentés comme les conséquences de la politique d'Extraction. La politique de Frustration se manifeste à la fois par un fort courant de radicalisme et la création de nombreux mouvements marginaux. L'esprit de clocher, pour sa part, se traduit par tin intérêt excessif pour la politique locale et par la domination soit des grandes corporations soit des autorités provinciales sur le gouvernement local. L'auteur appelle ces phénomènes: la politique de sublimation et la politique de dépendance.Il conclut en soutenant que les schémas politiques existants dans le nord-ouest ontarien ne connaitront probablement pas de changement significatif dans un avenir prévisible. Aucun parti politique n'est intéressé à des changements fondamentaux et la région ne dispose pas des ressources politiques, économiques, et sociales suffisantes pour exiger un tel changement.

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