Lehnherr H, Yarmolinsky M B
Laboratory of Biochemistry, National Cancer Institute, Bethesda, MD 20892-4255, USA.
Proc Natl Acad Sci U S A. 1995 Apr 11;92(8):3274-7. doi: 10.1073/pnas.92.8.3274.
Plasmid-encoded addiction genes augment the apparent stability of various low copy number bacterial plasmids by selectively killing plasmid-free (cured) segregants or their progeny. The addiction module of plasmid prophage P1 consists of a pair of genes called phd and doc. Phd serves to prevent host death when the prophage is retained and, should retention mechanisms fail, Doc causes death on curing. Doc acts as a cell toxin to which Phd is an antidote. In this study we show that host mutants with defects in either subunit of the ClpXP protease survive the loss of a plasmid that contains a P1 addiction module. The small antidote protein Phd is fully stable in these two mutant hosts, whereas it is labile in a wild-type host. We conclude that the role of ClpXP in the addiction mechanism of P1 is to degrade the Phd protein. This conclusion situates P1 among plasmids that elicit severe withdrawal symptoms and are able to do so because they encode both a cell toxin and an actively degraded macromolecule that blocks the synthesis or function of the toxin.
质粒编码的成瘾基因通过选择性杀死无质粒（治愈）的分离菌或其后代，增强了各种低拷贝数细菌质粒的表观稳定性。质粒原噬菌体P1的成瘾模块由一对名为phd和doc的基因组成。当原噬菌体被保留时，Phd可防止宿主死亡，并且如果保留机制失败，Doc会在治愈时导致宿主死亡。Doc作为一种细胞毒素，而Phd是其解毒剂。在本研究中，我们表明，ClpXP蛋白酶任一亚基存在缺陷的宿主突变体，在丢失含有P1成瘾模块的质粒后仍能存活。小解毒蛋白Phd在这两种突变宿主中完全稳定，而在野生型宿主中则不稳定。我们得出结论，ClpXP在P1成瘾机制中的作用是降解Phd蛋白。这一结论将P1置于会引发严重戒断症状的质粒之中，而这些质粒之所以能够引发症状，是因为它们既编码一种细胞毒素，又编码一种被主动降解的大分子，该大分子会阻断毒素的合成或功能。
