Abstract
SMC (structural maintenance of chromosomes) family members play essential roles in chromosome condensation, sister chromatid cohesion and DNA repair. It remains unclear how SMCs structure chromosomes and how their mechanochemical cycle regulates their interactions with DNA. Here we used single-molecule fluorescence microscopy to visualize how Bacillus subtilis SMC (BsSMC) interacts with flow-stretched DNAs. We report that BsSMC can slide on DNA, switching between static binding and diffusion. At higher concentrations, BsSMCs form clusters that condense DNA in a weakly ATP-dependent manner. ATP increases the apparent cooperativity of DNA condensation, demonstrating that BsSMC can interact cooperatively through their ATPase head domains. Consistent with these results, ATPase mutants compact DNA more slowly than wild-type BsSMC in the presence of ATP. Our results suggest that transiently static BsSMC molecules can nucleate the formation of clusters that act to locally condense the chromosome while forming long-range DNA bridges.
Highlights
SMC family members play essential roles in chromosome condensation, sister chromatid cohesion and DNA repair
Bacteriophage l genomic DNAs were tethered at one end to a polyethylene glycol (PEG)-passivated microfluidic flow cell via a biotin–neutravidin interaction
Genetic studies have clearly established that the bacterial SMC plays a critical role in chromosome organization, while biochemical experiments have demonstrated that SMC family members possess a number of DNA-modifying activities, including DNA condensation, bridging and supercoiling[35,36,38,39,40,41]
Summary
SMC (structural maintenance of chromosomes) family members play essential roles in chromosome condensation, sister chromatid cohesion and DNA repair. It remains unclear how SMCs structure chromosomes and how their mechanochemical cycle regulates their interactions with DNA. The conserved kleisin protein interacts with both SMC monomers and binds a second non-SMC component that varies among species. Deletion of the bacterial smc or either of its non-SMC subunits leads to defects in origin resolution, abnormal nucleoid structure, a significant increase in the percentage of anucleate cells and other phenotypes consistent with the SMC complex playing a vital role in chromosome organization and segregation[28,29]. The mechanism by which origin-proximal SMCs facilitate segregation of the chromosome remains unclear
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