Miki T, Chang Z T, Horiuchi T
J Mol Biol. 1984 Apr 25;174(4):627-46. doi: 10.1016/0022-2836(84)90087-1.
The genetic structure of the 42.84-43.6 F (BamHI-PstI) segment of the F plasmid, which contains all the F DNA sequences necessary for coupling cell division of F+ bacteria with plasmid DNA replication, was analyzed by isolating a series of amber mutants. Two cistrons were found in this region and they were designated letA and letD (an abbreviation for lethal mutation). The letA and letD cistrons were mapped on the 42.84-43.35 F (BamHI- XmaI ) segment and the 43.07-43.6 F (HincII-PstI) segment, respectively, and are presumed to correspond to the first (43.04-43.26 F) and second (43.26-43.57 F) open reading frames, respectively, which were found in this region by nucleotide sequencing. The letD gene product acts to inhibit cell division of the host bacteria and to induce prophages in lysogenic bacteria, whereas the letA gene product acts to suppress the activity of the letD gene product. Taking into consideration the fact that the 42.84-43.6 F segment carries all the F plasmid genes necessary for coupling cell division with plasmid DNA replication, and that the expression of the genes is likely to be controlled by plasmid DNA replication, we constructed the following hypothesis. Before completion of plasmid DNA replication, LetD protein acts to prevent cell division of the host bacteria. When plasmid DNA replication is completed, synthesis of LetA protein (and also LetD protein) takes place and the LetA protein synthesized acts to suppress the activity of LetD protein and make the cell ready for cell division. Actual cell division will take place when replication of both chromosomal and plasmid DNA is completed and the termination protein of the chromosome and the LetA protein of F plasmid are both synthesized. When cell division takes place LetA protein is consumed, and as a result LetD protein becomes active and prevents cell division until the next round of DNA replication is completed.
F质粒42.84 - 43.6F（BamHI - PstI）片段的遗传结构包含F +细菌细胞分裂与质粒DNA复制偶联所需的所有F DNA序列，通过分离一系列琥珀突变体对其进行了分析。在该区域发现了两个顺反子，分别命名为letA和letD（致死突变的缩写）。letA和letD顺反子分别定位在42.84 - 43.35F（BamHI - XmaI）片段和43.07 - 43.6F（HincII - PstI）片段上，推测分别对应于通过核苷酸测序在该区域发现的第一个（43.04 - 43.26F）和第二个（43.26 - 43.57F）开放阅读框。letD基因产物作用于抑制宿主细菌的细胞分裂并诱导溶原性细菌中的原噬菌体，而letA基因产物作用于抑制letD基因产物的活性。考虑到42.84 - 43.6F片段携带细胞分裂与质粒DNA复制偶联所需的所有F质粒基因，并且这些基因的表达可能受质粒DNA复制控制，我们构建了以下假说。在质粒DNA复制完成之前，LetD蛋白作用于阻止宿主细菌的细胞分裂。当质粒DNA复制完成时，LetA蛋白（以及LetD蛋白）开始合成，合成的LetA蛋白作用于抑制LetD蛋白的活性并使细胞为细胞分裂做好准备。当染色体和质粒DNA的复制都完成且染色体的终止蛋白和F质粒的LetA蛋白都合成时，实际的细胞分裂将发生。当细胞分裂发生时，LetA蛋白被消耗，结果LetD蛋白变得活跃并阻止细胞分裂，直到下一轮DNA复制完成。
