Jensen R B, Gerdes K
Department of Molecular Biology, Odense University, Denmark.
Mol Microbiol. 1995 Jul;17(2):205-10. doi: 10.1111/j.1365-2958.1995.mmi_17020205.x.
Bacterial plasmids are stabilized by a number of different mechanisms. Here we describe the molecular aspects of a group of plasmid-encoded gene systems called the proteic killer gene systems. These systems mediate plasmid maintenance by selectively killing plasmid-free cells (post-segregational killing or plasmid addiction). The group includes ccd of F, parD/pem of R1/R100, parDE of RP4/RK2, and phd/doc of P1. All of these systems encode a stable toxin and an unstable antidote. The antidotes prevent the lethal action of their cognate toxins by forming tight complexes with them. The antidotes are degraded by cellular proteases. Thus, the different decay rates of the toxins and antidotes seem to be the molecular basis of toxin activation in plasmid-free cells. The operons encoding the toxins and antidotes are autoregulated at the level of transcription either by a complex formed by the toxins and the cognate antidotes or by the antidote alone. The cellular targets of the killer proteins have been determined to be DNA gyrase in the case of ccd of F and DnaB in the case of parD of R1. Surprisingly, the Escherichia coli chromosome encodes at least two of these peculiar gene systems.
细菌质粒可通过多种不同机制得以稳定。在此，我们描述一类被称为蛋白质杀伤基因系统的质粒编码基因系统的分子层面情况。这些系统通过选择性杀死无质粒细胞（后分离杀伤或质粒成瘾）来介导质粒的维持。该类系统包括F质粒的ccd、R1/R100质粒的parD/pem、RP4/RK2质粒的parDE以及P1噬菌体的phd/doc。所有这些系统均编码一种稳定毒素和一种不稳定解毒剂。解毒剂通过与同源毒素形成紧密复合物来阻止其致死作用。解毒剂会被细胞蛋白酶降解。因此，毒素和解毒剂不同的降解速率似乎是无质粒细胞中毒素激活的分子基础。编码毒素和解毒剂的操纵子在转录水平上要么由毒素与同源解毒剂形成的复合物，要么由单独的解毒剂进行自动调控。已确定在F质粒的ccd情况下，杀伤蛋白的细胞靶点是DNA回旋酶；在R1质粒的parD情况下，靶点是DnaB。令人惊讶的是，大肠杆菌染色体至少编码其中两个奇特的基因系统。
