Crosa J H, Luttropp L K, Falkow S
J Bacteriol. 1976 Apr;126(1):454-66. doi: 10.1128/jb.126.1.454-466.1976.
Replicating deoxyribonucleic acid (DNA) molecules of plasmid RSF1040, a deletion mutant of the conjugative R plasmid R6K, appear in the electron microscope as partially supercoiled structures with two open circular branches of equal size, although open structures with three branches, two branching points and no supercoiled regions (theta structures) were also found at a lower frequency. The partially supercoiled molecules sediment more rapidly than native covalently closed circular DNA in neutral sucrose gradients and band at a position intermediate between covalently closed circular and open circular DNA in CsClethidium bromide gradients. Electron microscope measurements of the linear EcoRI-treated replicative intermediates indicate that replication can be initiated at two sites (origins) on the plasmid DNA molecule located at about 23% (alpha) and 39% (beta) of the total genome length from an EcoRI end designated arbitrarily as the "left-hand" end of the molecule. The overall replication of RSF1040 is asymmetrically bidirectional. Replication from the alpha origin proceeds first to the "right" to a unique termination site located some 55% of the total genome length from the left-hand end of the molecule. At this point replication proceeds from the alpha origin to the "left" (i.e., opposite to the original direction of replication) until replication of the molecule is completed. Replication also proceeds from the beta origin asymmetrically to the unique terminus site.
质粒RSF1040是接合型R质粒R6K的缺失突变体，其脱氧核糖核酸（DNA）分子在电子显微镜下呈现为部分超螺旋结构，带有两个大小相等的开放环状分支，不过也发现频率较低的具有三个分支、两个分支点且无超螺旋区域的开放结构（θ结构）。在中性蔗糖梯度中，部分超螺旋分子的沉降速度比天然共价闭合环状DNA更快，并且在溴化乙锭-CsCl梯度中，其条带位置介于共价闭合环状DNA和开放环状DNA之间。对经EcoRI处理的线性复制中间体进行电子显微镜测量表明，复制可在质粒DNA分子上的两个位点（起始点）起始，这两个位点分别位于从任意指定为分子“左手”端的EcoRI末端起占总基因组长度约23%（α）和39%（β）处。RSF1040的整体复制是不对称双向的。从α起始点开始的复制首先向“右”进行，到达位于从分子左手端起约占总基因组长度55%处的一个独特终止位点。此时，复制从α起始点向“左”进行（即与复制的原始方向相反），直至分子复制完成。复制也从β起始点不对称地向独特的终止位点进行。
