# 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. 

