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: Hha contains several surface exposed, positively charged residues that we have shown are critical for function (Figure 6a). The Hha mutant R14A/R17A, which lacks several of these residues, folds properly, associates with H-NS normally, but fails to augment gene silencing in vivo. Given the position of the charged residues in the complex, and their critical role in gene silencing, we previously postulated that these residues contact DNA. In this work, we have assessed whether the Hha(R14A/R17A) mutant maintained the ability to induce collapse of the H-NS–DNA complex and found that it does not. Figure 4 shows several trajectories for DNA bound by the H-NS/Hha(R14A/R17A) complex, and is seen to remain stable for at least 30 min post application of a mechanical tension that caused the DNA bound by the H-NS/Hha complex to collapse. These results suggest that the positively charged surface of Hha plays a role in overcoming electrostatic repulsion of adjacent DNA segments thereby enabling H-NS to effectively collapse DNA.
Affiliation: Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, ON L5L 1C6, Canada.