In DNA transfer via type IV secretion system (T4SS), relaxase enzyme releases linear ssDNA in donor cells and recircularizes in recipient cells. Using VirB/D4 T4SS, Agrobacterium cells can transfer an IncQ‐type plasmid depending on Mob relaxase and a model T‐DNA plasmid depending on VirD2 relaxase. Mobilization to Escherichia coli of the former plasmid is much more efficient than that of the latter, whereas an entirely reverse relationship is observed in transfer to yeast. These data suggest that either plasmid recircularization or conversion of ssDNA to dsDNA in the recipient bacterial cells is a rate‐limiting step of the transfer. In this study, we examined involvement of exonuclease genes in the plasmid acceptability. By the VirD2‐dependent T‐DNA plasmid, E. coli sbcDΔ and sbcCΔ mutant strains produced threefold more exconjugants, and a sbcDΔ xseAΔ mutant strain yielded eightfold more exconjugants than their wild‐type strain. In contrast to the enhancing effect on the VirD2‐mediated transfer, the mutations exhibited a subtle effect on the Mob‐mediated transfer. These results support our working hypothesis that VirD2 can transport its substrate ssDNA efficiently to recipient cells and that recipient nucleases degrade the ssDNA because VirD2 has some defect(s) in the circularization and completion of complementary DNA synthesis.

中文翻译：

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增强的农杆菌介导的转化揭示了核酸外切酶VII和SbcCD减弱了受体细菌中外源质粒DNA的安装。

在通过IV型分泌系统（T4SS）进行的DNA转移中，松弛酶在供体细胞中释放线性ssDNA，并在受体细胞中重新环化。使用VirB / D4 T4SS，农杆菌细胞可以根据Mob松弛酶转移IncQ型质粒，而根据VirD2松弛酶转移模型T-DNA质粒。前者质粒向大肠杆菌的动员效率比后者高得多，而在转移至酵母中观察到完全相反的关系。这些数据表明，质粒重新环化或受体细菌细胞中ssDNA到dsDNA的转化是转移的限速步骤。在这项研究中，我们检查了核酸外切酶基因在质粒可接受性中的参与。通过依赖VirD2的T-DNA质粒E. colisbcDΔ和sbcCΔ突变菌株产生的准分子比三倍多，而sbcDΔxseAΔ突变菌株产生的准分子比其野生型多八倍。与对VirD2介导的转移的增强作用相反，这些突变对Mob介导的转移表现出微妙的影响。这些结果支持了我们的工作假设：VirD2可以有效地将其底物ssDNA转运至受体细胞，并且受体核酸酶降解ssDNA，因为VirD2在环化和互补DNA合成完成方面存在一些缺陷。