Moreau P L
Laboratory of Enzymology, C.N.R.S., Gif-sur-Yvette, France.
J Mol Biol. 1987 Apr 20;194(4):621-34. doi: 10.1016/0022-2836(87)90239-7.
Overproduction of single-stranded DNA-binding protein (SSB) in Escherichia coli led to a decrease in the basal level of repressor LexA. Expression of the LexA-controlled genes was increased differentially, depending on the affinity of the LexA repressor for each promoter: expression of the recA and sfiA genes was increased 5-fold and 1.5-fold, respectively. Despite only a slight effect on expression of sfiA, which codes for an inhibitor of cell division, bacteria overproducing SSB produced elongated cells. In fact, the effect on cell shape appeared to be essentially independent of the expression of the sfiA and recA genes. Bacteria overproducing SSB were therefore phenotypically similar to bacteria partially starved of thymine, in which filamentation results from both sfiA-dependent and sfiA-recA-independent pathways. These data indicate that excess SSB acts primarily by perturbing DNA replication, thereby favoring gratuitous activation of RecA protein to promote cleavage of LexA protein. When bacteria overproducing SSB were exposed to a DNA-damaging agent such as ultraviolet light or mitomycin C, the recA and sfiA genes were fully induced. Induction of the sfiA gene occurred, however, at higher doses in bacteria overproducing SSB protein than in bacteria with normal levels of SSB. Whereas the efficiency of excision repair was apparently increased by excess SSB, the efficiency of post-replication recombinational repair was reduced as judged by a decrease in the recombination proficiency between a prophage and ultraviolet-irradiated heteroimmune infecting phage. Following induction of ssb+ bacteria with mitomycin C, the cellular content of SSB was slightly increased. These results provide evidence that SSB modulates RecA protein-dependent activities in vivo. It is proposed that SSB favors the formation of short complexes of RecA protein and single-stranded DNA that mediate cleavage of the LexA and lambda repressors, while it delays the formation of long nucleoprotein filaments, thereby slowing down RecA-promoted recombinational events in uninduced as well as in induced bacteria.
大肠杆菌中单链DNA结合蛋白（SSB）的过量产生导致阻遏蛋白LexA的基础水平下降。LexA控制的基因的表达根据LexA阻遏蛋白对每个启动子的亲和力不同而有差异地增加：recA和sfiA基因的表达分别增加了5倍和1.5倍。尽管对编码细胞分裂抑制剂的sfiA的表达只有轻微影响，但过量产生SSB的细菌产生了伸长的细胞。事实上，对细胞形状的影响似乎基本上与sfiA和recA基因的表达无关。因此，过量产生SSB的细菌在表型上类似于部分胸腺嘧啶饥饿的细菌，在后者中，丝状化是由依赖sfiA和不依赖sfiA-recA的途径导致的。这些数据表明，过量的SSB主要通过干扰DNA复制起作用，从而有利于RecA蛋白的无端激活以促进LexA蛋白的切割。当过量产生SSB的细菌暴露于DNA损伤剂如紫外线或丝裂霉素C时，recA和sfiA基因被完全诱导。然而，在过量产生SSB蛋白的细菌中，sfiA基因的诱导发生在比具有正常SSB水平的细菌更高的剂量下。尽管过量的SSB明显提高了切除修复的效率，但根据原噬菌体与紫外线照射的异源免疫感染噬菌体之间重组能力的降低判断，复制后重组修复的效率降低了。用丝裂霉素C诱导ssb +细菌后，SSB的细胞含量略有增加。这些结果提供了证据表明SSB在体内调节RecA蛋白依赖性活性。有人提出，SSB有利于RecA蛋白和单链DNA形成短复合物，介导LexA和λ阻遏蛋白的切割，同时它延迟长核蛋白丝的形成，从而减缓未诱导和诱导细菌中RecA促进的重组事件。
