Salt Concentration and Force Affect HU-DNA Interaction

Abstract

HU is one of the most abundant proteins in bacterial nucleoid and participates in nucleoid compaction and regulation. We used magnetic tweezers to study the dependence of DNA condensation by HU on force, salt and HU concentration. DNA bending exhibited only flexible hinge behavior at 150 mM and 200 mM NaCl, which may be considered physiological levels. No binding was observed at 300 mM NaCl. We tracked the disassociation of HU-DNA complexes in real time and found HU binding to be fully reversible in salt concentration above 100 mM NaCl. The 90% disassociation lifetime, t0.9, extended when the initial HU concentration in which the complexes formed was increased. If the salt concentration was raised while keeping the initial HU concentration and pulling force fixed, however, the t0.9 decreased. Taking 150 nM HU and 0.08 pN force for example, the average t0.9 was 233.0 minutes for 100 mM NaCl, 41.6 minutes for 150 mM, and 6.1 minutes for 200 mM. In addition, if the pulling force was increased from 0.08 pN to 0.28 pN, the t0.9 decreased by an amount dependent on the initial HU concentration. Our results suggested that HU-DNA association and disassociation can be regulated by a combination of mechanical tension, salt and HU concentration.