Lydersen B K, Pettijohn D E
Chromosoma. 1977 Jul 8;62(3):199-215. doi: 10.1007/BF00286044.
The structure of the bacterial chromosome was investigated after introducing breaks in the DNA with gamma irradiation. It is demonstrated that irradiation of the chromosome in the cell prior to isolation results in partial unfolding of the isolated condensed DNA, while irradiation of the chromosome after it is released from the cell has no demonstrable effect on DNA folding. The results indicate that RNA/DNA interactions which stabilize DNA folds are unstable when breaks are introduced in the DNA prior to isolation of the chromosome. It is suggested that the supercoiled state of the DNA is required for the initial stabilization of some of the critical RNA/DNA interaction in the isolated nucleoid. However, some of these interactions are not affected by irradiation of the cells. Remnant supercoiling in partially relaxed chromosomes containing a limited number of DNA breaks has the same superhelical density as the unirradiated chromosome. This suggests that restraints on rotation of the packaged DNA are formed prior to the physical unwinding which occurs at the sites of the radiation induced DNA breads. - Analysis of the in vitro irradiated chromosomes shows that there are 100 +/- 30 domains of supercoiling per genome equivalent of DNA. The introduction of up to 50 double-strand breaks per nucleoid does not influence rotor speed effects of the sedimentation coefficient of the chromosome.
在用γ射线照射使DNA产生断裂后，对细菌染色体的结构进行了研究。结果表明，在分离前对细胞内的染色体进行照射会导致分离出的浓缩DNA部分展开，而在染色体从细胞中释放后进行照射对DNA折叠没有明显影响。结果表明，在分离染色体之前DNA产生断裂时，稳定DNA折叠的RNA/DNA相互作用是不稳定的。有人提出，DNA的超螺旋状态是分离的类核中一些关键RNA/DNA相互作用初始稳定所必需的。然而，其中一些相互作用不受细胞照射的影响。含有有限数量DNA断裂的部分松弛染色体中的残余超螺旋与未照射的染色体具有相同的超螺旋密度。这表明，在辐射诱导的DNA断裂位点发生物理解旋之前，就形成了对包装DNA旋转的限制。- 对体外照射的染色体分析表明，每个基因组当量的DNA有100±30个超螺旋结构域。每个类核引入多达50个双链断裂不会影响染色体沉降系数的转子速度效应。
