Sex and Sensitivity: the (In)fidelity of DNA Homologous Recombination

Homologous recombination plays a key role in the evolution of organisms, from bacteria to man. While recombination generates genetic diversity by reshuffling genes and enabling the import of new ones, it must maintain at the same time the functionality of proteins. After an introduction t recombination and its biological significance, I will address two outstanding physics problems: first, how does a small tract of DNA entering a cell find the address of an homologous segment along a genome within a short time, and second, what is the accuracy or fidelity of molecular recognition. To shed light on these problems, I will present results of an in vitro study of RecA-promoted homologous recombination between short DNA oligomers with different degrees of heterology, using fluorescence resonant energy transfer (FRET) techniques. The experiments show that RecA can detect single sequence mismatches between the participating DNA strands at the initial stages of recombination, and that the efficiency of recombination is strongly dependent on the location and distribution of mismatches. Furthermore, we find that there is a characteristic DNA lengthscale above which the sensitivity to heterology decreases sharply. The experiments allow us to estimate the lifetime of complexes formed during homology search. The exquisite sensitivity to mismatches and their location during the initial stages of recombination support a mechanism for homology recognition that can be modeled as a kinetic proofreading cascade. These findings illustrate how non-equilibrium physics ideas are necessary to account for the extraordinary fidelity required to maintain life.