New helicase mechanism to regulate recombination

DNA recombination process is regarded as a double edged sword. Cells use recombination to repair breaks in a double strand DNA. However if not controlled carefully, recombination in an undesired context can lead to genomic instability which can result in cancer  in humans, for example. The group of Taekjip Ha in collaboration with Timothy M. Lohman at Washington University in St. Louis, has revealed a molecular mechanism for how the DNA motor protein, PcrA helicase can control DNA recombination which can occur when DNA replication is stalled (Park et al. Cell 142, 544-555, 2010).  Using Single-Molecule FRET techniques, CPLC student, Jeehae Park, found that PcrA can processively pull the ssDNA from the duplex junction, and thereby effectively dismantle filaments formed from RecA, the mediator of recombination. Molecular details of this activity were further investigated via an alternating excitation technique developed by graduate student, Kyungsuk Lee, and the role and the conformation of the subdomains within the protein machinery were identified.

This study has also answered more fundamental questions on how such biological motor proteins work. PcrA is a prototypical DNA motor protein that converts chemical energy of ATP hydrolysis into the mechanical energy of directional movement along the DNA. Our single-molecule approach revealed that molecular heterogeneity exists in the dynamics among the population of molecules, and when this heterogeneity is removed, clear kinetics are revealed;  the motor operates in one-nucleotide uniform steps. This information was previously inaccessible and therefore was not measured precisely even with the most advanced biochemical techniques.