Tang X B, Womble D D, Rownd R H
Department of Molecular Biology, The Medical School, Northwestern University, Chicago, Illinois 60611.
J Bacteriol. 1989 Oct;171(10):5290-5. doi: 10.1128/jb.171.10.5290-5295.1989.
By transformation of dnaA null mutant host cells that are suppressed either by an rnh mutation or by chromosomal integration of a mini-R1 plasmid, it was shown that replication of miniplasmids composed of the NR1 minimal replicon had no absolute dependence upon DnaA protein. In addition, the suppression of the dnaA null mutation by the integrated mini-R1, which is an IncFII relative of NR1, was found to be sensitive to the expression of IncFII-specific plasmid incompatibility. This suggests that the integrative suppression by mini-R1 is under the control of the normal IncFII plasmid replication circuitry. Although NR1 replication had no absolute requirement for DnaA, the copy numbers of NR1-derived miniplasmids were lower in dnaA null mutants, and the plasmids exhibited a much reduced stability of inheritance during subculture in the absence of selection. This suggests that DnaA protein may participate in IncFII plasmid replication in some auxiliary way, such as by increasing the efficiency of formation of an open initiation complex at the plasmid replication origin. Such an auxiliary role for DnaA in IncFII replication would be different from that for replication of most other plasmids examined, for which DnaA has been found to be either essential or unimportant.
通过对dnaA缺失突变宿主细胞进行转化，这些细胞可被rnh突变或mini-R1质粒的染色体整合所抑制，结果表明由NR1最小复制子组成的微型质粒的复制对DnaA蛋白没有绝对依赖性。此外，发现由与NR1同属IncFII的整合型mini-R1对dnaA缺失突变的抑制作用对IncFII特异性质粒不相容性的表达敏感。这表明mini-R1的整合抑制受正常IncFII质粒复制电路的控制。虽然NR1复制对DnaA没有绝对需求，但在dnaA缺失突变体中，源自NR1的微型质粒的拷贝数较低，并且在无选择的继代培养过程中，这些质粒的遗传稳定性大大降低。这表明DnaA蛋白可能以某种辅助方式参与IncFII质粒复制，例如通过提高在质粒复制起点处形成开放起始复合物的效率。DnaA在IncFII复制中的这种辅助作用与大多数其他已检测质粒的复制情况不同，对于其他质粒，DnaA已被发现要么是必需的，要么是不重要的。
