Nordström K, Molin S, Aagaard-Hansen H
Plasmid. 1980 Sep;4(2):215-27. doi: 10.1016/0147-619x(80)90011-6.
The stability of inheritance of plasmid R1drd-19 was tested. The copy number of the plasmid was determined in two different ways: As the ratio between covalently closed circular DNA and chromosomal DNA, and by quantitative determination of single-cell resistance to ampicillin. In the latter case, strains carrying the R1 ampicillin transposon Tn3 on prophage lambda was used as standard. The values were transformed to copy number per cell by using the Cooper-Helmstetter model for chromosome replication as well as by determination of chromosomal DNA per cell by the diphenylamine method. The copy number was found to be five to six per cell (or about four per newborn cell). Nevertheless, plasmid R1drd-19 was found to be completely stably inherited. This stability was shown not to be due to retransfer of the plasmid by the R1 conjugation system, since transfer-negative derivatives of the plasmid were also completely stably inherited. Smaller derivatives of plasmid R1drd-19 were found to be lost at a frequency of about 1.5% per cell generation. The copy-number control was not affected in these miniplasmids, since their copy numbers were the same as that of the full size plasmid. Quantitatively, the instability of the miniplasmids was in accord with random partitioning. It is, therefore, suggested that the plasmid R1drd-19 carries genetic information for partitioning (par) of plasmid copies at cell division, and that the par mechanism is distinct from the copy number control (cop) system. Finally, the par gene maps on the resistance transfer part of the plasmid, but far away from the origin of replication and the so-called basic replicon; this is in accord with the approximate location of the repB gene (Yoshikawa, 1974, J. Bacteriol., 118, 1123-1131).
对质粒R1drd - 19的遗传稳定性进行了测试。质粒的拷贝数通过两种不同方式测定：作为共价闭合环状DNA与染色体DNA的比率，以及通过定量测定单细胞对氨苄青霉素的抗性。在后一种情况下，携带噬菌体λ上R1氨苄青霉素转座子Tn3的菌株用作标准。通过使用库珀 - 赫尔姆施泰特染色体复制模型以及通过二苯胺法测定每个细胞的染色体DNA，将这些值转化为每个细胞的拷贝数。发现每个细胞的拷贝数为五到六个（或每个新生细胞约四个）。然而，发现质粒R1drd - 19是完全稳定遗传的。这种稳定性并非由于质粒通过R1接合系统的重新转移，因为该质粒的转移阴性衍生物也完全稳定遗传。发现质粒R1drd - 19的较小衍生物以约每细胞世代1.5％的频率丢失。这些微型质粒中的拷贝数控制未受影响，因为它们的拷贝数与全长质粒相同。从数量上看，微型质粒的不稳定性符合随机分配。因此，表明质粒R1drd - 19携带在细胞分裂时对质粒拷贝进行分配（par）的遗传信息，并且par机制与拷贝数控制（cop）系统不同。最后，par基因定位于质粒的抗性转移部分，但远离复制起点和所谓的基本复制子；这与repB基因的大致位置一致（吉川，1974年，《细菌学杂志》，118，1123 - 1131）。
