Mec1 is activated at the onset of normal S phase by low dNTP pools impeding DNA replication

The Mec1 and Rad53 kinases play a central role during acute replication stress in budding yeast. They are also essential for viability in normal growth conditions, but the signal that activates the Mec1-Rad53 pathway in the absence of exogenous insults is currently unknown. Here, we show that this pathway is active at the onset of normal S phase because dNTP levels present in G1 phase may be not sufficient to support processive DNA synthesis and impede DNA replication. This activation can be suppressed experimentally by increasing dNTP levels in G1 phase. Moreover, we show that unchallenged cells entering S phase in the absence of Rad53 undergo irreversible fork collapse and mitotic catastrophe. Together, these data indicate that cells use suboptimal dNTP pools to detect the onset of DNA replication and activate the Mec1-Rad53 pathway, which in turn maintains functional forks and triggers dNTP synthesis, allowing the completion of DNA replication.

麦克斯1（Mec1）和拉德53（Rad53）激酶在芽殖酵母的急性复制压力期间扮演着核心角色。它们在正常生长条件下对于细胞的生存也是至关重要的，然而，在没有外源性损伤的情况下激活麦克斯1-拉德53途径的信号目前尚不明确。在本研究中，我们发现该途径在正常S期开始时即处于激活状态，因为G1期存在的脱氧核糖核苷酸（dNTP）水平可能不足以支持连续的DNA合成并阻碍DNA复制。通过实验增加G1期的dNTP水平可以抑制这种激活。此外，我们还发现，在没有拉德53的情况下进入S期的未受挑战的细胞将经历不可逆的复制叉崩溃和有丝分裂灾难。综上所述，这些数据表明，细胞利用次优的dNTP库来检测DNA复制的开始并激活麦克斯1-拉德53途径，进而维持功能性的复制叉并触发dNTP的合成，从而允许DNA复制的完成。