Brcić-Kostić K, Salaj-Smic E, Marsić N, Kajić S, Stojiljković I, Trgovcević Z
Ruder Bosković Institute, Zagreb, Croatia, Yugoslavia.
Mol Gen Genet. 1991 Aug;228(1-2):136-42. doi: 10.1007/BF00282458.
The DNA of a gene 2 mutant (T4 2-) of phage T4 is degraded by RecBCD enzyme in the bacterial cytoplasm. Under normal conditions, recBCD+ cells are therefore incapable of supporting the growth of phage T4 2-. Only if the nucleolytic activity of RecBCD enzyme is absent from the cytoplasm are T4 2(-)-infected bacteria able to form plaques. We found that recBCD+ cells can form plaques if, before infection with T4 2-, they have been exposed to gamma radiation. It is suggested that gamma ray-induced lesions of the bacterial DNA (e.g., double-strand breaks) bind RecBCD enzyme. This binding enables the enzyme to begin to degrade the bacterial chromosome, but simultaneously prevents its degradative action on the ends of minor DNA species, such as unprotected infecting phage chromosomes. Degradation of the chromosomal DNA, which occurs during the early postirradiation period, ceases about 60 min after gamma ray exposure. The reappearance of the nucleolytic action of RecBCD enzyme on T4 2- DNA accompanies the cessation of degradation of bacterial DNA. Both, this cessation and the reappearance of the nucleolytic action of ReCBCD enzyme on T4 2- DNA depend on a functional recA gene product. These results suggest that postirradiation DNA degradation is controlled by the recA-dependent removal of RecBCD enzyme from the damaged chromosome. By making use of the temperature-sensitive mutant recB270, we showed that RecBCD-mediated repair of gamma ray-induced lesions occurs during the early postirradiation period, i.e. during postirradiation DNA degradation. It is shown that the RecD subunit of RecBCD enzyme also participates in this repair.
噬菌体T4基因2突变体（T4 2-）的DNA在细菌细胞质中会被RecBCD酶降解。因此，在正常情况下，recBCD+细胞无法支持噬菌体T4 2-的生长。只有当细胞质中不存在RecBCD酶的核酸酶活性时，被T4 2（-）感染的细菌才能形成噬菌斑。我们发现，如果recBCD+细胞在感染T4 2-之前受到γ射线照射，它们就能形成噬菌斑。这表明γ射线诱导的细菌DNA损伤（如双链断裂）会结合RecBCD酶。这种结合使该酶开始降解细菌染色体，但同时阻止其对少量DNA分子末端（如未受保护的感染噬菌体染色体末端）的降解作用。染色体DNA的降解发生在照射后的早期阶段，在γ射线照射后约60分钟停止。RecBCD酶对T4 2- DNA的核酸酶活性的再次出现与细菌DNA降解的停止同时发生。细菌DNA降解的停止以及RecBCD酶对T4 2- DNA核酸酶活性的再次出现都依赖于功能性recA基因产物。这些结果表明，照射后DNA的降解是由recA依赖的RecBCD酶从受损染色体上的去除所控制的。通过利用温度敏感突变体recB270，我们表明RecBCD介导的γ射线诱导损伤的修复发生在照射后的早期阶段，即在照射后DNA降解期间。结果表明，RecBCD酶的RecD亚基也参与了这种修复。
