Femtonewton Forces Can Control Protein-Meditated DNA Looping Yih-Fan Chen, 1 J. N. Milstein, 2 and Jens-Christian Meiners 2, 3 $^{1}$Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109 $^{2}$Department of Physics, University of Michigan, Ann Arbor, Michigan 48109 $^{3}$LSA Biophysics, University of Michigan, Ann Arbor, Michigan 48109 (Dated: November 14, 2018) Abstract We show that minuscule entropic forces, on the order of 100 fN, can prevent the formation of DNA loops–a ubiquitous means of regulating the expression of genes. We observe a tenfold decrease in the rate of LacI-mediated DNA loop formation when a tension of 200 fN is applied to the substrate DNA, biasing the thermal fluctuations that drive loop formation and breakdown events. Conversely, once looped, the DNA-protein complex is insensitive to applied force. Our measurements are in excellent agreement with a simple polymer model of loop formation in DNA, and show that an anti-parallel topology is the preferred LacI-DNA loop conformation for a generic loop-forming construct. 1 arXiv:1001.0546v1 [q-bio.BM] 4 Jan 2010