Weinert T A, Hartwell L H
Department of Genetics, University of Washington, Seattle 98195.
Genetics. 1993 May;134(1):63-80. doi: 10.1093/genetics/134.1.63.
In eucaryotes a cell cycle control called a checkpoint ensures that mitosis occurs only after chromosomes are completely replicated and any damage is repaired. The function of this checkpoint in budding yeast requires the RAD9 gene. Here we examine the role of the RAD9 gene in the arrest of the 12 cell division cycle (cdc) mutants, temperature-sensitive lethal mutants that arrest in specific phases of the cell cycle at a restrictive temperature. We found that in four cdc mutants the cdc rad9 cells failed to arrest after a shift to the restrictive temperature, rather they continued cell division and died rapidly, whereas the cdc RAD cells arrested and remained viable. The cell cycle and genetic phenotypes of the 12 cdc RAD mutants indicate the function of the RAD9 checkpoint is phase-specific and signal-specific. First, the four cdc RAD mutants that required RAD9 each arrested in the late S/G2 phase after a shift to the restrictive temperature when DNA replication was complete or nearly complete, and second, each leaves DNA lesions when the CDC gene product is limiting for cell division. Three of the four CDC genes are known to encode DNA replication enzymes. We found that the RAD17 gene is also essential for the function of the RAD9 checkpoint because it is required for phase-specific arrest of the same four cdc mutants. We also show that both X- or UV-irradiated cells require the RAD9 and RAD17 genes for delay in the G2 phase. Together, these results indicate that the RAD9 checkpoint is apparently activated only by DNA lesions and arrests cell division only in the late S/G2 phase.
在真核生物中，一种名为检查点的细胞周期控制机制可确保有丝分裂仅在染色体完全复制且任何损伤得到修复后才会发生。芽殖酵母中该检查点的功能需要RAD9基因。在此，我们研究了RAD9基因在12个细胞分裂周期（cdc）突变体停滞中的作用，这些温度敏感致死突变体在限制温度下会在细胞周期的特定阶段停滞。我们发现，在四个cdc突变体中，cdc rad9细胞在转移至限制温度后无法停滞，而是继续细胞分裂并迅速死亡，而cdc RAD细胞则停滞并保持存活。这12个cdc RAD突变体的细胞周期和遗传表型表明，RAD9检查点的功能具有阶段特异性和信号特异性。首先，四个需要RAD9的cdc RAD突变体在转移至限制温度后，当DNA复制完成或接近完成时，均在S/G2期后期停滞；其次，当CDC基因产物限制细胞分裂时，每个突变体都会留下DNA损伤。已知四个CDC基因中的三个编码DNA复制酶。我们发现RAD17基因对于RAD9检查点的功能也至关重要，因为它是相同四个cdc突变体阶段特异性停滞所必需的。我们还表明，无论是X射线还是紫外线照射的细胞，在G2期延迟都需要RAD9和RAD17基因。总之，这些结果表明，RAD9检查点显然仅由DNA损伤激活，并且仅在S/G2期后期停滞细胞分裂。
