A biomechanical mechanism for initiating DNA packaging.
Bottom Line: The bacterial chromosome is under varying levels of mechanical stress due to a high degree of crowding and dynamic protein-DNA interactions experienced within the nucleoid.The nucleoid structuring protein H-NS is a key regulator of DNA condensation and gene expression in enterobacteria and its activity in vivo is affected by the accessory factor Hha.Our results imply that H-NS requires Hha to condense bacterial DNA and that this condensation could be triggered by the level of mechanical tension experienced along different regions of the chromosome.
Affiliation: Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, ON L5L 1C6, Canada.Show MeSH
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Mentions: Figure 2 shows the dynamical change in RMS motion of multiple tethers, which are plotted together so that t = 0 refers to the moment when each tethered microsphere is released from the optical trap. At a 100 nM concentration of H-NS and Hha, an applied tension triggers a dynamic collapse of the nucleoprotein complex that occurs on the order of 10–15 min. The final RMS are found to be between 1/2 and 1/3 of the pre-stretched RMS. At 200 nM, however, the complex shows much greater compaction, on similar time scales, with some complexes displaying an RMS motion an order-of-magnitude less than before the force was applied. Note that occasionally the tether length shortens because the bead sticks to the surface, but these are rapid, discrete events easily filtered from the data. The relatively flat curves at the top of each figure display the behavior of control tethers that were not stretched by the optical tweezers and show that the protein–DNA complex does not condense in the absence of an applied force. Once collapsed, the protein–DNA complex appears quite stable. We repeatedly tried to extend the collapsed tethers with our optical tweezers, exerting forces of up to 50 pN, but were unable to reverse the collapse.
Affiliation: Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, ON L5L 1C6, Canada.