McGuinness, C. | Search bioRxiv

bioRxiv2026

Increased replication-associated single-stranded DNA promotes formaldehyde-induced mutagenesis.

Formaldehyde (FA) is an environmentally abundant and endogenously produced aldehyde that has been shown to cause DNA damage, mutagenesis, and carcinogenesis. Several studies have demonstrated that FA induces guanine mutations resulting in a mutation signature like SBS40. In this work, we demonstrate that replication defects generating single-stranded DNA (ssDNA) caused by the downregulation of the major replicative polymerases results in elevated FA mutagenesis. We found that loss of Mrc1 (CLASPIN) resulted in a high accumulation of ssDNA and FA mutagenesis, and that these phenotypes were not dependent on Mrc1s checkpoint activity. Loss of DNA-protein crosslink repair results in elevated FA sensitivity with no alteration to mutagenesis, likely due to the inability of the fork to bypass unprocessed protein adducts. Finally, we show that FA-induced mutagenesis is dependent on Pol {zeta}-mediated translesion synthesis, while deficiencies in the template switching pathway do not alter error-free bypass of FA adducts. Overall, our work points towards replication-associated ssDNA as a major substrate for FA-induced damage and elucidates the pathways that function to prevent FA mutagenesis at replication forks.

genetics↗

bioRxiv2022

Dynamic microtubules slow down during their shrinkage phase

Microtubules are dynamic polymers that undergo stochastic transitions between growing and shrinking phases. The structural and chemical properties of these phases remain poorly understood. The transition from growth to shrinkage, termed catastrophe, is not a first-order reaction but is rather a multi-step process whose frequency increases with the growth time: the microtubule ages as the older microtubule tip becomes more unstable. Aging shows that the growing phase is not a single state but comprises several substates of increasing instability. To investigate whether the shrinking phase is also multi-state, we characterized the kinetics of microtubule shrinkage following catastrophe using an in vitro reconstitution assay with purified tubulins. We found that the shrinkage speed is highly variable across microtubules and that the shrinkage speed of individual microtubules slows down over time by as much as several fold. The shrinkage slowdown was observed in both fluorescently labeled and unlabeled microtubules as well as in microtubules polymerized from tubulin purified from different species, suggesting that the shrinkage slowdown is a general property of microtubules. These results indicate that microtubule shrinkage, like catastrophe, is time-dependent and that the shrinking microtubule tip passes through a succession of states of increasing stability. We hypothesize that the shrinkage slowdown is due to destabilizing events that took place during growth which led to multi-step catastrophe. This suggests that the aging associated with growth is also manifest during shrinkage with the older, more unstable growing tip being associated with a faster depolymerizing shrinking tip. Statement of SignificanceThe dynamics of the microtubule cytoskeleton is crucial for several functions in eukaryotic cells. Microtubule dynamics is traditionally described by constant growth and shrinkage speeds with first order transitions between the growth and shrinkage phases. However, catastrophe, the transition from growth to shrinkage, increases with microtubule age and is not a first order process. In contrast to the common assumption that microtubules shrink with constant speed, here we show that shrinking microtubule tips undergo step-wise slowdown during depolymerization. Our results suggest that microtubule shrinkage, like catastrophe, is a multi-step process. This finding is important for understanding the molecular nature of microtubule dynamic instability and how microtubule shrinkage can be modulated by microtubule associated proteins.

biophysics↗