Compaction and resolution of replicated chromosomes into morphologically and spatially distinct sister chromatids is essential for faithful DNA segregation in all organisms, but the molecular mechanisms that underlie these processes are poorly understood. In bacteria, chromosome segregation is largely driven by DNA compaction, which is thought to occur by the orderly folding of chromosomes along adjacent DNA segments drawing replicated sisters in on themselves and away from each other. In virtually all organisms, Structural Maintenance of Chromosomes (SMC) condensin complexes play a central role in this process but how these ring-shaped ATPase function has remained unclear. In Bacillus subtilis, condensin rings are topologically loaded onto the chromosome adjacent to the origin of replication by the partitioning protein ParB bound to centromeric parS sites. Using chromosome conformation capture (Hi-C) and ChIP-seq, we discovered that these complexes then travel down the left and right chromosome arms all the way to the terminus while tethering the two arms together. Our findings support a generalizable model in which SMC complexes act along adjacent DNA segments by processively enlarging DNA loops. In this model, these ring-shaped complexes encircle the DNA flanking their loading site, tethering the duplexes together. As these tethers move away from their loading sites they generate loops. Loop- formation ensures that these complexes act along adjacent DNA segments and therefore resolve rather than tangle sister chromosomes. In B. subtilis, processive loop enlargement centered on origin-proximal parS sites draws sister origins in on themselves and away from each other. De novo loop formation along chromosome arms in eukaryotes can also explain how condensin complexes compact and resolve sister chromatids during mitosis and provides a mechanism for the formation of transcriptionally insulated domains (also called topologically associated domains or TADs) by SMC cohesin complexes during interphase. Our studies on the B. subtilis SMC complex highlights the importance of using simple model systems to study conserved cell biological processes. The experiments described in this proposal build on our recent discoveries and preliminary findings to define how these broadly conserved complexes generate DNA loops; how these ring- shaped tethers are removed when they reach the replication terminus; and the role of condensin in remodeling the bacterial chromosome during the replication-segregation cycle.
In this proposal we investigate how Structural Maintenance of Chromosome (SMC) complexes remodel chromosomes by processively enlarging DNA loops. Mutations in the SMC cohesin complex are associated with human multi-spectrum developmental abnormalities termed cohesinopathies and mutations in SMC condensin complexes have been associated with T-cell lymphomas and colon cancer. A greater understanding of SMC complex function has the potential to provide insight into the underlying mechanisms of cohesinopathies and to identify possible therapeutic targets for a subset of cancers.
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