The survival of cells with defective Smc5/6 complexes under replication stress depends on a delicate balance between DNA damage checkpoint signaling and the regulation of homologous recombination (HR) intermediates. While both pathways are essential, their relative contributions differ depending on the duration and intensity of stress. The smc6-P4 mutant exhibits profound sensitivity to methyl methanesulfonate (MMS), characterized by elevated levels of X-shaped recombination intermediates (X-mols) and impaired Rad53 phosphorylation—a hallmark of defective checkpoint activation. Genetic suppression by deletion of the helicase Mph1 has long been attributed to reduced X-mol accumulation. However, our findings reveal that Mph1 also induces persistent Rad53 hyperphosphorylation, indicating a dual role in enhancing checkpoint signaling.
To determine whether checkpoint hyperactivation alone can rescue smc6-P4 viability, we used two independent approaches to bypass the need for endogenous DNA damage signals. First, we introduced the TEL1-hy909 allele, which confers constitutive Tel1 kinase activity. This resulted in strong Rad53 phosphorylation in smc6-P4 cells without reducing X-mol levels. Second, we engineered an inducible system where Ddc1 and Ddc2 were fused to LacI-GFP modules and targeted to chromosomal LacO arrays. Upon galactose induction, forced proximity of these sensor proteins activated the Mec1 checkpoint even in the absence of DNA lesions, leading to sustained Rad53 phosphorylation. Both interventions restored checkpoint function in smc6-P4 cells and significantly improved survival following transient MMS exposure—mirroring the effect of mph1 deletion.
Despite this improvement, neither strategy conferred resistance to chronic MMS stress. In contrast, mph1 deletion robustly protected smc6-P4 cells under prolonged exposure. This discrepancy suggested that while enhanced checkpoint signaling aids short-term tolerance, it cannot compensate for the underlying defect in HR intermediate resolution. To test this directly, we removed the checkpoint sensor protein Mec3 from smc6-P4 mph1 double mutants. This abolished Rad53 hyperphosphorylation and attenuated checkpoint response, yet mph1 still fully suppressed MMS sensitivity. Furthermore, X-mol levels remained unchanged in these triple mutants, confirming that checkpoint attenuation did not affect HR intermediate turnover. These results indicate that the protective effect of mph1 under chronic stress is primarily due to its ability to reduce recombination intermediates, not checkpoint activation.
Our data support a two-phase model of smc6 mutant survival: during transient replication stress, enhanced checkpoint signaling stabilizes stalled forks and delays cell cycle progression, allowing time for repair and proper segregation.Apixaban impurity 13 In Vitro Under chronic stress, however, persistent HR intermediates become toxic, and their removal becomes the dominant factor for viability.Omeprazole Bacterial Thus, the Smc5/6 complex functions not only in dismantling recombination intermediates but also in modulating checkpoint responses, with these roles being separable.PMID:34808126 This functional distinction may extend to other HR-related mutants such as sgs1 and esc2, which similarly accumulate X-mols and exhibit checkpoint defects. By decoupling checkpoint activation from HR regulation, our study provides a framework for understanding how cells prioritize different mechanisms of genome stability maintenance under varying stress conditions.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com