Boreham D R, Mitchel R E
Atomic Energy of Canada Limited, Radiation Biology Branch, Ontario.
Radiat Res. 1993 Sep;135(3):365-71.
Saccharomyces cerevisiae and Chlamydomonas reinhardtii respond to a sublethal exposure of ionizing radiation by increasing their resistance to killing by a second exposure. We demonstrate here that the two lower eukaryotes apparently achieve this by different mechanisms. We have shown that induced radioresistance in yeast results from increased capacity for recombinational repair, which we believe to occur in G2-phase haploid cells by recombination between homologous chromosomes. This is not possible in G1-phase haploid cells, which lack a second copy of DNA. Haploid C. reinhardtii cells, however, show induced resistance when irradiated asynchronously or in the G1 phase of the cell cycle. We have shown previously that the development of radiation resistance in yeast is proportional to the magnitude of the inducing dose and clearly demonstrates an oxygen effect. There was no oxygen effect for induced radiation resistance in C. reinhardtii cells, but induction remained proportional to dose. In yeast we have reported that both increased radioresistance and thermotolerance are inducible by a heat shock. Here, C. reinhardtii showed induced thermotolerance but no induced radioresistance in response to a heat stress. We have also determined previously that the induced recombinational DNA repair system in yeast recognizes alkylation lesions and therefore confers increased resistance to mutation by MNNG. In these experiments, C. reinhardtii induced for radioresistance were not more resistant to MNNG mutagenesis. These data indicate that haploid C. reinhardtii has a unique DSB repair mechanism. We propose that one possible mechanism may involve chloroplast DNA in a cooperative chloroplast/nuclear recombinational repair process.
酿酒酵母和莱茵衣藻在受到亚致死剂量的电离辐射后，对再次照射的杀伤抵抗力会增强。我们在此证明，这两种低等真核生物实现此现象的机制明显不同。我们已表明，酵母中诱导产生的辐射抗性源于重组修复能力的增强，我们认为这发生在G2期单倍体细胞中，通过同源染色体之间的重组实现。这在G1期单倍体细胞中是不可能的，因为它们缺乏DNA的第二个拷贝。然而，单倍体莱茵衣藻细胞在异步照射或处于细胞周期的G1期时会表现出诱导抗性。我们之前已表明，酵母中辐射抗性的发展与诱导剂量的大小成正比，并且清楚地显示出氧效应。莱茵衣藻细胞中诱导产生的辐射抗性没有氧效应，但诱导仍与剂量成正比。在酵母中我们曾报道，辐射抗性增加和耐热性都可由热激诱导产生。在此，莱茵衣藻对热应激表现出诱导耐热性，但没有诱导辐射抗性。我们之前还确定，酵母中诱导的重组DNA修复系统能识别烷基化损伤，因此赋予对MNNG诱变的抗性增加。在这些实验中，诱导产生辐射抗性的莱茵衣藻对MNNG诱变并不更具抗性。这些数据表明，单倍体莱茵衣藻具有独特的双链断裂修复机制。我们提出一种可能的机制可能涉及叶绿体DNA参与叶绿体/细胞核协同重组修复过程。
