Meijer W J, van der Lelie D, Venema G, Bron S
Department of Genetics, Centre for Biological Sciences, Kerklaan, Haren, The Netherlands.
Plasmid. 1995 Mar;33(2):79-89. doi: 10.1006/plas.1995.1010.
The effects of the single-strand origins (SSOs) of plasmid pMV158 on (i) the conversion of its single-stranded (ss) replication intermediates to double-stranded (ds) plasmid DNA and (ii) its maintenance were analyzed. The rolling-circle plasmid pMV158, which replicates via ssDNA intermediates, contains two single-strand origins (SSOs) of replication, palA and palU. In this paper the results obtained with Bacillus subtilis are described; complementary studies with Lactococcus lactis are presented in the accompanying paper (Meijer et al., 1995). While in L. lactis both SSOs are functional as ssDNA conversion signal, only palU appeared to be active B. subtilis. Similar to the situation in L. lactis, the accumulation of large amounts of ssDNA resulted in a severe decrease in plasmid maintenance in B. subtilis. In the latter bacterium large amounts of ssDNA were only accumulated, however, when plasmids lacking a functional SSO were propagated in RecA mutant strains. In wild-type RecA strains these plasmids accumulated only modest amounts of ssDNA and they were maintained at fairly stable levels. The results suggest that in B. subtilis a RecA-mediated alternative pathway exists for the conversion of ssDNA which can improve plasmid maintenance. In addition to ssDNA accumulation and the antagonizing role of RecA therein, two other plasmid regions were shown to affect pMV158 maintenance in B. subtilis. One was the mob gene region, which had a negative effect on plasmid maintenance, and the other the palA type SSO. Although palA was not functional as an ssDNA conversion signal in B. subtilis, its presence had a positive effect on pMV158 maintenance.
分析了质粒pMV158的单链复制起点（SSO）对（i）其单链（ss）复制中间体转化为双链（ds）质粒DNA的影响以及（ii）对其稳定性的影响。滚环质粒pMV158通过ssDNA中间体进行复制，含有两个复制单链起点（SSO），即palA和palU。本文描述了在枯草芽孢杆菌中获得的结果；在随附论文中给出了与乳酸乳球菌的补充研究（Meijer等人，1995年）。在乳酸乳球菌中，两个SSO均作为ssDNA转化信号起作用，但在枯草芽孢杆菌中只有palU似乎具有活性。与乳酸乳球菌中的情况类似，大量ssDNA的积累导致枯草芽孢杆菌中质粒稳定性严重下降。然而，在后者的细菌中，只有当缺乏功能性SSO的质粒在RecA突变菌株中繁殖时，才会积累大量ssDNA。在野生型RecA菌株中，这些质粒仅积累少量ssDNA，并且它们以相当稳定的水平得以维持。结果表明，在枯草芽孢杆菌中存在一条由RecA介导的用于ssDNA转化的替代途径，该途径可提高质粒的稳定性。除了ssDNA积累以及RecA在其中的拮抗作用外，还显示出另外两个质粒区域会影响pMV158在枯草芽孢杆菌中的稳定性。一个是mob基因区域，它对质粒稳定性有负面影响，另一个是palA类型的SSO。尽管palA在枯草芽孢杆菌中不作为ssDNA转化信号起作用，但其存在对pMV158的稳定性有积极影响。
