Homologous recombination (HR) is a ubiquitous DNA double-strand break (DSB) repair mechanism that promotes cell survival. It entails a potentially genome-wide homology search step, carried out along a conserved RecA/Rad51-ssDNA nucleoprotein filament (NPF) assembled on each DSB ends 1-3 . This search is subdued to NPF-dsDNA collision probability, dictated in part by chromatin conformation 2,4 . In contrast to the extensive knowledge about chromatin composition and mobility changes elicited by the DNA damage checkpoint (DDC) 5-7 , whether, how, and to which extent a DSB impacts spatial chromatin organization, and whether this organization in turns influences the homology search process, remains ill-defined 8,9 . Here we characterize two layers of spatial chromatin reorganization following DSB formation in S. cerevisiae. While cohesin folds chromosomes into cohesive arrays of 10-20 kb long chromatin loops as cells arrest inG2/M 10,11 , the DSB-flanking regions locally interact in a resection-and 9-1-1 clamp-dependent manner, independently of cohesin and HR proteins. This local structure blocks cohesin progression, constraining the extending NPF at loop base. Functionally this organization promotes side-specific cis DSB-dsDNA interactions that scales with loop expansion span, and provides a kinetic advantage for identification of intra-over inter-chromosomal homologies. We propose that cohesins regulate homology search by promoting cis dsDNA over-sampling, both upon loop expansion-coupled unidimensional dsDNA scanning, .

Selective protein adsorption is a key challenge for the development of biosensors, separation technologies, and smart materials for medicine and biotechnologies. In this work, a strategy was developed for selective protein adsorption, based on the use of mixed polymer brushes composed of poly­(ethylene oxide) (PEO), a protein-repellent polymer, and poly­(acrylic acid) (PAA), a weak polyacid whose conformation changes according to the pH and ionic strength of the surrounding medium. A mixture of lysozyme (Lyz), human serum albumin (HSA), and human fibrinogen (Fb) was used to demonstrate the success of this strategy. Polymer brush formation and protein adsorption were monitored by quartz crystal microbalance, whereas protein identification after adsorption from the mixture was performed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) with principal component analysis and gel electrophoresis with silver staining. For the ToF-SIMS measurements, adsorption was first performed from single-protein solutions in order to identify characteristic peaks of each protein. Next, adsorption was performed from the mixture of the three proteins. Proteins were also desorbed from the brushes and analyzed by gel electrophoresis with silver staining for further identification. Selective adsorption of Lyz from a mixture of Lyz/HSA/Fb was successfully achieved at pH 9.0 and ionic strength of 10–3 M, while Lyz and HSA, but not Fb, were adsorbed at ionic strength 10–2 M and pH 9.0. The results demonstrate that by controlling the ionic strength, selective adsorption can be achieved from protein mixtures on PEO/PAA mixed brushes, predominantly because of the resulting control on electrostatic interactions. In well-chosen conditions, the selectively adsorbed proteins can also be fully recovered from the brushes by a simple ionic strength stimulus. The developed systems will find applications as responsive biointerfaces in the fields of separation technologies, biosensing, drug delivery, and nanomedicine.