Lehnherr H, Maguin E, Jafri S, Yarmolinsky M B
Laboratory of Biochemistry, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892.
J Mol Biol. 1993 Oct 5;233(3):414-28. doi: 10.1006/jmbi.1993.1521.
P1 lysogens of Escherichia coli carry the prophage as a stable low copy number plasmid. The frequency with which viable cells cured of prophage are produced is about 10(-5) per cell per generation. Here we show that a significant part of this remarkable stability can be attributed to a plasmid-encoded mechanism that causes death of cells that have lost P1. In other words, the lysogenic cells appear to be addicted to the presence of the prophage. The plasmid withdrawal response depends on a gene named doc (death on curing), encoding a 126 amino acid protein. Expression of doc is not SOS-inducing and killing by Doc is recA-independent. In cells that retain P1 the killing is prevented by the product of a gene named phd (prevent host death), encoding a 73 amino acid protein. The genes phd and doc have been cloned and expressed from a 0.7 kb segment of P1 DNA. The two genes constitute an operon and the synthesis of Doc appears to be translationally coupled to that of Phd. Homologs of the P1 addiction genes are found elsewhere, but phd and doc are unrelated to previously described genes of other plasmids that also cause an apparent increase in plasmid stability by post-segregational killing.
大肠杆菌的P1溶原菌携带原噬菌体作为一种稳定的低拷贝数质粒。每代每个细胞产生不含原噬菌体的活细胞的频率约为10^(-5)。我们在此表明，这种显著稳定性的很大一部分可归因于一种质粒编码机制，该机制导致失去P1的细胞死亡。换句话说，溶原细胞似乎对原噬菌体的存在有依赖性。质粒去除反应取决于一个名为doc（治愈时死亡）的基因，该基因编码一种126个氨基酸的蛋白质。Doc的表达不诱导SOS，且Doc介导的杀伤不依赖recA。在保留P1的细胞中，一种名为phd（防止宿主死亡）的基因产物可阻止这种杀伤，phd基因编码一种73个氨基酸的蛋白质。phd和doc基因已从P1 DNA的一个0.7 kb片段中克隆并表达。这两个基因构成一个操纵子，Doc的合成似乎与Phd的合成在翻译上偶联。P1成瘾基因的同源物在其他地方也有发现，但phd和doc与先前描述的其他质粒基因无关，那些质粒基因也通过后分离杀伤导致质粒稳定性明显增加。
