Chromosome length influences replication-induced topological stress.

During chromosome duplication the parental DNA molecule becomes overwound, or positively supercoiled, in the region ahead of the advancing replication fork. To allow fork progression, this superhelical tension has to be removed by topoisomerases, which operate by introducing transient DNA breaks. Positive supercoiling can also be diminished if the advancing fork rotates along the DNA helix, but then sister chromatid intertwinings form in its wake. Despite these insights it remains largely unknown how replication-induced superhelical stress is dealt with on linear, eukaryotic chromosomes. Here we show that this stress increases with the length of Saccharomyces cerevisiae chromosomes. This highlights the possibility that superhelical tension is handled on a chromosome scale and not only within topologically closed chromosomal domains as the current view predicts. We found that inhibition of type I topoisomerases leads to a late replication delay of longer, but not shorter, chromosomes. This phenotype is also displayed by cells expressing mutated versions of the cohesin- and condensin-related Smc5/6 complex. The frequency of chromosomal association sites of the Smc5/6 complex increases in response to chromosome lengthening, chromosome circularization, or inactivation of topoisomerase 2, all having the potential to increase the number of sister chromatid intertwinings. Furthermore, non-functional Smc6 reduces the accumulation of intertwined sister plasmids after one round of replication in the absence of topoisomerase 2 function. Our results demonstrate that the length of a chromosome influences the need of superhelical tension release in Saccharomyces cerevisiae, and allow us to propose a model where the Smc5/6 complex facilitates fork rotation by sequestering nascent chromatid intertwinings that form behind the replication machinery.