Bendich A J
Department of Botany, University of Washington, Seattle, WA 98195-5325, USA.
Biochimie. 2001 Feb;83(2):177-86. doi: 10.1016/s0300-9084(00)01209-8.
The circular concept of the bacterial chromosome was based initially on experiments involving conjugation mapping and autoradiographic imaging of DNA. This view was then supported by DNA fragment mapping, genome sequencing, and the analysis of linear DNA produced by a single cleavage of chromosomal DNA. A circular chromosome is also indicated by the existence of a mechanism for segregating dimeric chromosomes produced by recombination and the replication of DNA on both sides of the replication terminus. The evidence for circularity is reviewed here and found to be compatible with either a circular or a linear chromosomal DNA molecule. Moving pictures of ethidium-stained DNA revealed most chromosomal DNA as a rosette form with loops emanating from a dense node or as a network of strands lacking a node. This description applies to Escherichia coli, Agrobacterium tumefaciens, Pyrococcus endeavorii, Vibrio cholerae, and both the linear-mapping chromosome of Streptomyces lividans and its circular-mapping derivative. Networks without nodes were found for two linear-mapping Borrelia species. For the E. coli chromosome, open-form circles of various sizes were found only at extremely low frequency. The node of the rosette was reduced in size or eliminated in recA mutants, as well as by treatment with either ribonuclease, topoisomerase IV, 1 M NaCl, or lysozyme. A model is presented for the bacterial chromosome in which the DNA is compacted by many points of strand association (including recombination junctions, tangles and knots) created during the repair of DNA damage that occurs many times in each chromosome replication cycle.
细菌染色体的环状概念最初是基于涉及接合图谱绘制和DNA放射自显影成像的实验。随后，DNA片段图谱分析、基因组测序以及对染色体DNA单次切割产生的线性DNA的分析也支持了这一观点。环状染色体的存在还通过一种机制得以体现，即分离由重组产生的二聚体染色体以及在复制终点两侧进行DNA复制。本文回顾了支持环状结构的证据，发现其与环状或线性染色体DNA分子均相符。溴化乙锭染色DNA的动态图像显示，大多数染色体DNA呈玫瑰花结形式，有环从一个密集节点发出，或者呈无节点的链状网络。这一描述适用于大肠杆菌、根癌土壤杆菌、顽强火球菌、霍乱弧菌，以及淡紫链霉菌的线性图谱染色体及其环状图谱衍生物。在两种线性图谱的疏螺旋体物种中发现了无节点的网络。对于大肠杆菌染色体，仅在极低频率下发现各种大小的开放形式环。玫瑰花结的节点在recA突变体中尺寸减小或消失，用核糖核酸酶、拓扑异构酶IV、1M氯化钠或溶菌酶处理也会出现这种情况。本文提出了一个细菌染色体模型，其中DNA通过在每个染色体复制周期中多次发生的DNA损伤修复过程中产生的许多链关联点（包括重组连接、缠结和结）而被压缩。
