Monday, January 9, 2012
"How do DNA-binding motor proteins navigate on crowded DNA?" Ilya J. Finkelstein Department of Biochemistry and Molecular Biophysics, Columbia University, New York, NY, 10032 In the cell, DNA-binding motor enzymes act on substrates occupied by other proteins, yet little is known regarding the inevitable collisions that must occur. Using nanofabricated curtains of DNA and real-time, multi-color single-molecule microscopy we visualized collisions between two model translocases and DNA-bound obstacles. We showed that RecBCD, a helicase involved in DNA repair, actively disrupts nucleoprotein complexes, including transcribing RNA polymerase (RNAP) in either head-to-head or head-to-tail orientations, Lac repressor, EcoRI(E111Q) and even nucleosomes. RecBCD did not pause during collisions and could push proteins thousands of base-pairs before ejecting them from DNA. Furthermore, FtsK, a hexameric dsDNA motor protein involved in bacterial chromosome resolution, is also able to push and displace DNA-bound protein obstacles such as RNAP and EcoRI(E111Q). Surprisingly, FtsK can also bypass roadblocks, possibly via a transient and partial disassembly of the motor subunits. I conclude with a mechanistic description of how RecBCD and FtsK act as powerful molecular wire-strippers to overwhelm potential roadblocks as they travel along DNA.