Abstract
Cohesin plays a critical role in sister chromatid cohesion, double-stranded DNA break repair and regulation of gene expression. However, the mechanism of how cohesin directly interacts with DNA remains unclear. We report single-molecule experiments analyzing the interaction of the budding yeast cohesin Structural Maintenance of Chromosome (SMC)1-SMC3 heterodimer with naked double-helix DNA. The cohesin heterodimer is able to compact DNA molecules against applied forces of 0.45 pN, via a series of extension steps of a well-defined size ≈130 nm. This reaction does not require ATP, but is dependent on DNA supercoiling: DNA with positive torsional stress is compacted more quickly than negatively supercoiled or nicked DNAs. Un-nicked torsionally relaxed DNA is a poor substrate for the compaction reaction. Experiments with mutant proteins indicate that the dimerization hinge region is crucial to the folding reaction. We conclude that the SMC1-SMC3 heterodimer is able to restructure the DNA double helix into a series of loops, with a preference for positive writhe.
Highlights
Structural Maintenance of Chromosomes (SMC) complexes are present in eukaryotes, prokaryotes and archaea, and are responsible for a variety of chromosome-organizing functions [1,2,3]
Eukaryote condensin SMCs play a key role in mitotic chromosome condensation [4], while eukaryote cohesin SMCs hold sister chromatids together during mitosis until they are degraded during anaphase [5,6,7]
We found that SMC1/3 can drive a stepwise DNA compaction reaction on nicked DNA
Summary
Structural Maintenance of Chromosomes (SMC) complexes are present in eukaryotes, prokaryotes and archaea, and are responsible for a variety of chromosome-organizing functions [1,2,3]. Eukaryote condensin SMCs play a key role in mitotic chromosome condensation [4], while eukaryote cohesin SMCs hold sister chromatids together during mitosis until they are degraded during anaphase [5,6,7]. Apart from its function of building sister chromatid cohesion, cohesin plays an essential role in double-stranded DNA break repair [8,9] and gene regulation, and mutations in cohesins and their regulators are associated with numerous human developmental diseases [10]. Gene-regulatory functions of cohesin follow from its involvement in maintenance of interphase chromatin ‘loop’ organization [11,12]. Despite it being established that cohesin plays a central role in chromatin organization, exactly how cohesins physically interact with chromosomal DNA is still an open question
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