Zhang S, Meyer R
Department of Microbiology, University of Texas, Austin 78712, USA.
Mol Microbiol. 1997 Aug;25(3):509-16. doi: 10.1046/j.1365-2958.1997.4861849.x.
The frequency of conjugal mobilization of plasmid R1162 is decreased approximately 50-fold if donor cells lack MobC, one of the plasmid-encoded proteins making up the relaxosome at the origin of transfer (oriT). The absence of MobC has several different effects on oriT DNA. Site- and strand-specific nicking by MobA protein is severely reduced, accounting for the lower frequency of mobilization. The localized DNA strand separation required for this nicking is less affected, but becomes more sensitive to the level of active DNA gyrase in the cell. In addition, strand separation is not efficiently extended through the region containing the nick site. These effects suggest a model in which MobC acts as a molecular wedge for the relaxosome-induced melting of oriT DNA. The effect of MobC on strand separation may be partially complemented by the helical distortion induced by supercoiling. However, MobC extends the melted region through the nick site, thus providing the single-stranded substrate required for cleavage by MobA.
如果供体细胞缺乏MobC（构成转移起始点oriT处松弛体的一种质粒编码蛋白），质粒R1162的接合转移频率会降低约50倍。MobC的缺失对oriT DNA有几种不同的影响。MobA蛋白介导的位点特异性和链特异性切口严重减少，这解释了转移频率较低的原因。这种切口所需的局部DNA链分离受影响较小，但对细胞中活性DNA促旋酶的水平变得更加敏感。此外，链分离不能有效地延伸通过包含切口位点的区域。这些效应提示了一个模型，其中MobC作为分子楔，促进松弛体诱导的oriT DNA解链。MobC对链分离的影响可能部分被超螺旋诱导的螺旋扭曲所补充。然而，MobC通过切口位点延伸解链区域，从而提供MobA切割所需的单链底物。
