Mechanism of DNA Sequence Alignment During Homologous Recombination
题目： Mechanism of DNA Sequence Alignment During Homologous Recombination
报告人：Dr. Zhi Qi
Department of Biochemistry & Molecular Biophysics, Columbia University, New York, NY
Homologous recombination (HR) promotes the exchange genetic information between different DNA molecules. During HR, members of the RecA/Rad51 family of recombinases must align and pair ssDNA with a homologous dsDNA template, but the physical basis for these early stages of recombination remain largely undefined. Here we use single-molecule imaging to visualize Rad51 as it attempts to align and pair homologous DNA sequences in real-time. We show that Rad51 uses a length-based recognition mechanism while interrogating dsDNA, enabling robust kinetic selection of 8-nucleotide (nt) tracts of microhomology, which confines the search to sites with a high probability of being the homologous target. Successful pairing with a 9th nucleotide coincides with an additional reduction in binding free energy and subsequent strand invasion occurs in precise 3-nt steps, reflecting the unusual base triplet organization of the presynaptic complex. These findings provide crucial new insights into the physical and evolutionary underpinnings of DNA recombination.