Saunders C W, Guild W R
Mol Gen Genet. 1980;180(3):573-8. doi: 10.1007/BF00268062.
Two plasmids from group B streptococcus were introduced into pneumococcus (Streptococcus pneumoniae) and examined for copy number, stability, and some features of the process by which they transform pneumococcal recipients. The 3.6 Mdal pMV158 (tet) was present at a minimum of 12 to 16 copies per chromosome and was never observed to be cured. The 20 Mdal pIP501 (cat erm) had a minimum copy number of 3 to 4 per chromosome and was lost spontaneously at a frequency near 0.03 per division. The presence of novobiocin increased this frequency 2 to 3-fold. Competence for chromosomal transformation and the membrane endonuclease needed for normal DNA entry were required for plasmid transformation. Plasmid transformants segregated transformed cells one generation ahead of chromosomal transformants. Both single and multiple hit components of the transformation reaction kinetics were observed, but the latter could not be seen in the presence of competing chromosomal DNA. The major of the transforming activity behaved as covalently closed circular DNA in dye-buoyancy gradients. Although most of the activity for both plasmids sedimented in sucrose gradients more rapidly than did monomeric closed circular DNA, a significant fraction was found at a position suggesting that it may have been due to monomeric plasmids.
将B组链球菌的两种质粒导入肺炎球菌（肺炎链球菌），并检测其拷贝数、稳定性以及它们转化肺炎球菌受体的过程的一些特征。3.6 Mdal的pMV158（tet）每个染色体至少有12至16个拷贝，从未观察到其被消除。20 Mdal的pIP501（cat erm）每个染色体的最小拷贝数为3至4个，并且以接近每代0.03的频率自发丢失。新生霉素的存在使该频率增加2至3倍。质粒转化需要染色体转化能力和正常DNA进入所需的膜内切酶。质粒转化子比染色体转化子提前一代分离出转化细胞。观察到转化反应动力学的单 hit 和多 hit 成分，但在存在竞争性染色体DNA的情况下看不到后者。在染料浮力梯度中，大部分转化活性表现为共价闭合环状DNA。尽管两种质粒的大部分活性在蔗糖梯度中的沉降速度比单体闭合环状DNA快，但在一个位置发现了相当一部分，这表明它可能是由于单体质粒。
