Abstract
Camptothecin‐induced locking of topoisomerase 1 on DNA generates a physical barrier to replication fork progression and creates topological stress. By allowing replisome rotation, absence of the Tof1/Csm3 complex promotes the conversion of impending topological stress to DNA catenation and causes camptothecin hypersensitivity. Through synthetic viability screening, we discovered that histone H4 K16 deacetylation drives the sensitivity of yeast cells to camptothecin and that inactivation of this pathway by mutating H4 K16 or the genes SIR1‐4 suppresses much of the hypersensitivity of tof1∆ strains towards this agent. We show that disruption of rDNA or telomeric silencing does not mediate camptothecin resistance but that disruption of Sir1‐dependent chromatin domains is sufficient to suppress camptothecin sensitivity in wild‐type and tof1∆ cells. We suggest that topoisomerase 1 inhibition in proximity of these domains causes topological stress that leads to DNA hypercatenation, especially in the absence of the Tof1/Csm3 complex. Finally, we provide evidence of the evolutionarily conservation of this mechanism.
Highlights
Separation of the two parental DNA strands during DNA replication creates positive supercoiling ahead of the replication fork
Our results show that the disruption of chromatin domains bearing deacetylated H4 K16 rescues the camptothecin hypersensitivity of tof1Δ and csm3Δ cells, suggesting that the increased sister chromatid catenation generated in the absence of these proteins promotes camptothecin toxicity
To better understand the roles of the Tof1/Csm3 complex during DNA replication, we investigated the basis for the camptothecin hypersensitivity of TOF1- or CSM3-deleted cells
Summary
Separation of the two parental DNA strands during DNA replication creates positive supercoiling ahead of the replication fork. In contrast to Top, Top is essential in yeast cells because a certain amount of catenation is generated even in wild-type cells, possibly because Top cannot relieve topological stress between replisomes converging towards replication termination zones [8]. Consistent with this model, increased fork rotation has been observed when replication forks approach stable forkpausing structures, such as centromeres, tRNA genes, inactive replication origins [9], and potentially retrotransposon long terminal repeats (LTRs) and transcriptionally repressed chromatin [10,11]
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