Strand asymmetry influences mismatch resolution during a single-strand annealing

Background: Biases of DNA repair can shape the nucleotide landscape of genomes at evolutionary timescales. The molecular mechanisms of those biases are still poorly understood because it is difficult to isolate the contributions of DNA repair from those of DNA damage. Results: Here, we develop a genome-wide assay whereby the same DNA lesion is repaired in different genomic contexts. We insert thousands of barcoded transposons carrying a reporter of DNA mismatch repair in the genome of mouse embryonic stem cells. Upon inducing a double-strand break between tandem repeats, a mismatch is generated if the break is repaired through single-strand annealing. The resolution of the mismatch showed a 60-80% bias in favor of the strand with the longest 3' flap. The location of the lesion in the genome and the type of mismatch had little influence on the bias. Instead, we observe a complete reversal of the bias when the longest 3' flap is moved to the opposite strand by changing the position of the double-strand break in the reporter. Conclusions: These results suggest that the processing of the double-strand break has a major influence on the repair of mismatches during a single-strand annealing.We acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC RGPIN-2020-06377), the Spanish Ministry of Economy, Industry and Competitiveness (“Centro de Excelencia Severo Ochoa 2013-2017”, Plan Estatal PGC2018-099807-B-I00), of the CERCA Programme/Generalitat de Catalunya, and of the European Research Council (Synergy Grant 609989). VOP was supported by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie programme (665385). We also acknowledge the support of the Spanish Ministry of Economy and Competitiveness (MEIC) to the EMBL partnershi

An experimental assay of the interactions of amino acids from orthologous sequences shaping a complex fitness landscape

Characterizing the fitness landscape, a representation of fitness for a large set of genotypes, is key to understanding how genetic information is interpreted to create functional organisms. Here we determined the evolutionarily-relevant segment of the fitness landscape of His3, a gene coding for an enzyme in the histidine synthesis pathway, focusing on combinations of amino acid states found at orthologous sites of extant species. Just 15% of amino acids found in yeast His3 orthologues were always neutral while the impact on fitness of the remaining 85% depended on the genetic background. Furthermore, at 67% of sites, amino acid replacements were under sign epistasis, having both strongly positive and negative effect in different genetic backgrounds. 46% of sites were under reciprocal sign epistasis. The fitness impact of amino acid replacements was influenced by only a few genetic backgrounds but involved interaction of multiple sites, shaping a rugged fitness landscape in which many of the shortest paths between highly fit genotypes are inaccessible.The work was supported by HHMI International Early Career Scientist Program (55007424), the MINECO (BFU2012-31329, BFU2012-37168, BFU2015-68351-P and BFU2015-68723-P), Spanish Ministry of Economy and Competitiveness Centro de Excelencia Severo Ochoa 2013-2017 grant (SEV-2012-0208), the Unidad de Excelencia María de Maeztu funded by the MINECO (MDM-2014-0370), Secretaria d'Universitats i Recerca del Departament d'Economia i Coneixement de la Generalitat AGAUR program (2014 SGR 0974), the CERCA Programme of the Generalitat de Catalunya, Russian Foundation for Basic Research grant (18-04-01173), the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie programme (665385) and the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013, ERC grant agreement 335980_EinME and Synergy Grant 609989). KSS was supported by EMBO long-term fellowship (ALTF 107-2016). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript