Kinetic properties of optogenetic site-specific DNA recombination by LiCre-loxP

Advances in optogenetics now allow to specifically modify the DNA of live cells with light. However, using these technologies successfully requires to know their properties in terms of sensitivity, efficiency, kinetics and mechanism. We previously developed an optogenetic tool made of a single chimeric protein called LiCre that enables the induction of specific changes in the genome with blue light via DNA recombination between loxP sites [1]. Here, we used in vitro and in vivo experiments combined with kinetic modeling to provide a deeper characterization of the photo-activated LiCre-loxP recombination reaction. We find that LiCre binds DNA with high affinity in absence of light stimulus and that this binding is cooperative although not as much as for the Cre recombinase from which LiCre was derived. In yeast, addition of riboflavin to the culture medium had no effect on LiCres efficiency, even when cells over-expressed riboflavin kinase, suggesting that abundance of the flavin mono-nucleotide co-factor is not limiting for the reaction. However, LiCres efficiency in yeast gradually increased when raising temperature from 20{degrees}C to 37{degrees}C. The recombination kinetics observed in live cells are best explained by a model where photo-activation of two or more DNA-bound LiCre units (happening in seconds) can produce (in several minutes) a functional recombination synapse. This model was able to capture the effect of a point mutation altering LiCres light cycle. This deeper understanding of the LiCre-loxP system provides additional knowledge for designing experiments where specific genetic changes are induced in live cells with light. AUTHOR SUMMARYUsing a technology called optogenetics, scientists are now able to change the DNA sequence of live cells by illuminating them with light. In theory, they can trigger in genetically-engineered organisms a mutation of interest in specific cells at a specific time. This practice, however, is not common because optogenetics relies on light-controlled enzymes that are recent and not well characterized. We previously developed one of these enzymes, called LiCre, which recognizes a specific piece of DNA. Following illumination with blue light, LiCre can switch on or off whatever gene is close by. Here, we combined experiments and computational modeling to better understand how, and how fast, LiCre works. We find that, although it is inactive in the dark, LiCre does not need photo-activation to bind to its DNA partner. We estimated the speed at which LiCre gets deactivated in the dark and the speed at which active LiCre molecules modify DNA. We also showed the effect of temperature and illumination dynamics on LiCres efficiency. These results will help design strategies where LiCre can be used to conduct genetic studies at high spatio-temporal resolution, or implement it in industrial and biomedical applications.