Low-Force DNA Condensation and Discontinuous High-Force Decondensation Reveal a Loop-Stabilizing Function of the Protein Fis

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Abstract
We report single-DNA-stretching experiments showing that the protein Fis, an abundant bacterial chromosome protein of E. coli, mediates a dramatic DNA condensation to zero length. This condensation occurs abruptly when DNA tension is reduced below a protein-concentration-dependent threshold f* < 1 pN. Following condensation, reopening under larger forces proceeds via a series of discrete jumps, indicating that Fis is able to stabilize DNA crossings. Our experiments suggest that Fis may play a role in vivo stabilizing the "loop-domain" structure of the bacterial chromosome.