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Research Funding:

Human Frontier Science Programme Organization (RGP0050/2002-C to LF)

Emory University Research Council-2006 and NIH R01, RGM084070A to LF

Emory College of Arts and Sciences Seed Grant

Determination of the number of proteins bound non-specifically to DNA


Journal Title:

Journal of Physics: Condensed Matter


Volume 22, Number 41


, Pages 414104-414104

Type of Work:

Article | Post-print: After Peer Review


We have determined the change in the number of proteins bound non-specifically to DNA as a function of applied force using force–extension measurements on tethered DNA. Using magnetic tweezers, single molecules of λ DNA were repeatedly stretched and relaxed in the absence and presence of 170 nM λ repressor protein (CI). CI binds to six specific sites of λ DNA with nanomolar affinity and also binds non-specifically with micromolar affinity. The force versus extension data were analyzed using a recently developed theoretical framework for quantitative determination of protein binding to the DNA. The results indicate that the non-specific binding of CI changes the force–extension relation significantly in comparison to that of naked DNA. The DNA tether used in our experiment would have about 640 bound repressors, if it was completely saturated with bound proteins. We find that as the pulling force on DNA is reduced from 4.81 to 0.13 pN, approximately 138 proteins bind to DNA, which is about 22% of the length of the tethered DNA. Our results show that 0.13 pN is not low enough to cause saturation of DNA by repressor and 4.81 pN is also not high enough to eliminate all the repressors bound to DNA. This demonstrates that the force–extension relation provides an effective approach for estimating the number of proteins bound non-specifically to a DNA molecule.

Copyright information:

© 2010 IOP Publishing Ltd

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