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Rapid pairing and subsequent resegregation of distant homologous loci enables double-strand break repair in bacteria

ABSTRACT: Double-strand breaks (DSBs) can lead to the loss of genetic information and cell death. Consequently, cells in all domains of life have evolved mechanisms to repair DSBs, including through homologous recombination. Although recombination has been well characterized, the spatial organization of this process in living cells remains poorly understood. Here, we introduced site-specific DSBs in Caulobacter crescentus, and then used time-lapse microscopy to visualize the homology search, DSB repair, and the resegregation of chromosomal DNA. Even loci tethered to opposite cell poles can efficiently release, pair to enable recombination-based repair, and then resegregate to their original locations. Resegregation occurs independent of DNA replication and without disrupting global chromosome organization. Origin-proximal regions are resegregated by the same machinery, ParABS, used to segregate undamaged chromosomes following DNA replication. In contrast, origin-distal regions efficiently resegregate after a DSB independent of ParABS, and likely without dedicated segregation proteins. Instead, we propose that a physical, spring-like force drives the resegregation of origin-distal loci after DSB repair.

ORGANISM(S): Caulobacter vibrioides NA1000

PROVIDER: GSE66811 | GEO | 2015/07/03

SECONDARY ACCESSION(S): PRJNA278008

REPOSITORIES: GEO

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Project description:Double-strand breaks (DSBs) can lead to the loss of genetic information and cell death. Consequently, cells in all domains of life have evolved mechanisms to repair DSBs, including through homologous recombination. Although recombination has been well characterized, the spatial organization of this process in living cells remains poorly understood. Here, we introduced site-specific DSBs in Caulobacter crescentus, and then used time-lapse microscopy to visualize the homology search, DSB repair, and the resegregation of chromosomal DNA. Even loci tethered to opposite cell poles can efficiently release, pair to enable recombination-based repair, and then resegregate to their original locations. Resegregation occurs independent of DNA replication and without disrupting global chromosome organization. Origin-proximal regions are resegregated by the same machinery, ParABS, used to segregate undamaged chromosomes following DNA replication. In contrast, origin-distal regions efficiently resegregate after a DSB independent of ParABS, and likely without dedicated segregation proteins. Instead, we propose that a physical, spring-like force drives the resegregation of origin-distal loci after DSB repair. Caulobacter cells were depleted of DnaA for 1.5 h before synchronization. Swarmer cells were then released into DnaA depleting conditions (without IPTG) and double-strand breaks were induced for 1 h by the addition of 500 ?M vanillate. For control sample, no vanillate was added. Formadehyde (Sigma) was then added to the final concentration of 1%. Formadehyde crosslinks protein-DNA and DNA-DNA together, thereby capturing the structure of the chromosome at the time of fixation. Fixation was performed at the cell density of OD600 = 0.2. The crosslinking reactions were allowed to proceed for 30 minutes at 25 °C before quenching with 2.5 M glycine at a final concentration of 0.125 M. Fixed cells were then pelleted by centrifugation and subsequently washed twice with 1x M2 buffer (6.1 mM Na2HPO4, 3.9 mM KH2PO4, 9.3 mM NH4Cl, 0.5 mM MgSO4, 10 ?M FeSO4, 0.5 mM CaCl2) before resuspending in 1x TE buffer (10 mM Tris-HCl pH 8.0 and 1 mM EDTA) to a final concentration of 107 cells per µl. Resuspended cells were then divided into 25 µl aliquots and stored at -80 °C for no more than 2 weeks. Each Hi-C experiment was performed using two of the 25 µL aliquots. Chromosome conformation capture with next-generation seqeuncing (Hi-C) was carried out exactly as described previously (Le et al., 2013 PMID: 24158908)

Project description:DNA double-strand breaks (DSBs) initiate meiotic recombination. Past DSB-mapping studies have used rad50S or sae2? mutants, which are defective in break processing, to accumulate DSBs, and report large (= 50 kb) “DSB-hot” regions that are separated by “DSB-cold” domains of similar size. Substantial recombination occurs in some DSB-cold regions, suggesting that DSB patterns are not normal in rad50S or sae2? mutants. We therefore developed novel methods that detect DSBs using ssDNA enrichment and microarray hybridization, and that use background-based normalization to allow cross-comparison between array datasets, to map genome-wide the DSBs that accumulate in processing-capable, repair-defective dmc1î and dmc1î rad51î mutants. DSBs were observed at known hotspots, but also in most previously-identified “DSB-cold” regions, including near centromeres and telomeres. While about 40% of the genome is DSB-cold in rad50S mutants, analysis of meiotic ssDNA from dmc1? shows that most of these regions have significant DSB activity. Thus, DSBs are distributed much more uniformly than was previously believed. Southern-blot assays of DSBs in selected regions in dmc1?, rad50S and wild-type cells confirm these findings. Comparisons of DSB signals in dmc1, dmc1 rad51, and dmc1 spo11 mutant strains identify Dmc1 as the primary strand transfer activity genome-wide, and Spo11-induced lesions as initiating all meiotic recombination. Keywords: DSB mapping, ChIP-chip, single strand DNA , BND cellulose We use two different strategies to map the genome-wide distribution of meiotic DSBs in the yeast Saccharomyces cerevisiae. The first is a chromatin immunoprecipitation (ChIP) based approach that targets the Spo11p protein, which remains covalently attached to DSB ends in the rad50S mutant background. The second approach involves BND cellulose enrichment of the single strand DNA (ssDNA) recombination intermediate formed by end-resection at DSB sites following Spo11p removal. We use dmc1 and dmc1 rad51 mutants that accumulates meiotic single strand DNA intermediates

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Rapid pairing and subsequent resegregation of distant homologous loci enables double-strand break repair in bacteria

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