Screaning of E.coli Genes That Participate in the Partitioning of F Plasmid DNA

F质粒的letA（ccdA）和letD（ccdB）基因位于复制必需序列的外部，有助于质粒在大肠杆菌细胞中的稳定维持。letD基因产物通过直接相互作用使DNA促旋酶的A亚基失活，从而抑制染色体DNA的分配和细胞分裂，而letA基因产物则逆转letD基因产物的抑制活性。为了鉴定参与这一过程的宿主因子，我们分析了逃避letD产物生长抑制的突变体及其抑制子，发现除了我们先前报道的groE基因之外，还有三个大肠杆菌基因tldD、tldE和zfiA参与。tldD和tldE突变使细胞耐受letD产物生长抑制，groES突变也是如此，而zfiA基因中的突变使tldD、tldE和groES突变体letD敏感。我们假设这些基因产物是与letD基因产物沿着调节DNA促旋酶活性的因子; zfiA基因产物用于抑制letD蛋白和DNA促旋酶A亚基之间的相互作用，而tldD、tldE和zfiA基因产物用于抑制zfiA基因产物的抑制活性。tldD、tldE和zfiA基因分别位于大肠杆菌染色体上的70.4、96.0和58.2分钟处，分别编码相对分子质量为51000、48000和6800的蛋白质。tldD是一个新基因，而tldE和zfiA基因被证明分别是pmbA基因（产生Microcin B17）和csrA基因（碳链调节因子）。

The letA (ccdA) and letD (ccdB) genes of the F plasmid, located just outside the sequence essential for replication, contribute to stable maintenance of the plasmid in Escherichia coli cells. The letD gene product acts to inhibit partitioning of chromosomal DNA and cell division by inactivating the A subunit of DNA gyrase by a direct interaction, whereas the letA gene product acts to reverse the inhibitory activity of the letD gene product. To identify the host factor (s) involved in this process, we analyzed such mutants that escaped the letD product growth inhibition and their suppressor, and found that three E.coli genes, tldD,tldE and zfiA participate, in addition to the groE genes we reported previously. tldD and tldE mutations made cells tolerant to the letD product growth inhibition as did groES mutations, while the mutation in the zfiA gene made tldD,tldE and groES mutants letD sensitive. We assume that these gene products are factors that modulate activity of DNA gyrase along with the letD gene product ; zfiA gene product acts to inhibit interaction between the letD protein and the A subunit of DNA gyrase, while the tldD,tldE and zfiA gene products act to suppress the inhibitory activity of the zfiA gene product. The tldD,tldE and zfiA genes are located at 70.4,96.0 and 58.2 minutes on the E coli chromosome, respectively, and code for proteins with relative molecular masses of 51,000,48,000 and 6800, respectively. The tldD is a novel gene, but the tldE and zfiA genes proved to be the pmbA gene (production of Microcin B17) and the csrA gene (carbon strage regulator), respectively.