Li L Y, Shoemaker N B, Salyers A A
Department of Microbiology, University of Illinois, Urbana 61801, USA.
J Bacteriol. 1995 Sep;177(17):4992-9. doi: 10.1128/jb.177.17.4992-4999.1995.
Many Bacteroides clinical isolates contain large conjugative transposons, which excise from the genome of a donor and transfer themselves to a recipient by a process that requires cell-to-cell contact. It has been suggested that the transfer intermediate of the conjugative transposons is a covalently closed circle, which is transferred by the same type of rolling circle mechanism used by conjugative plasmids, but the transfer origin of a conjugative transposon has not previously been localized and characterized. We have now identified the transfer origin (oriT) region of one of the Bacteroides conjugative transposons, TcrEmr DOT, and have shown that it is located near the middle of the conjugative transposon. We have also identified a 16-kbp region of the conjugal transposon which is necessary and sufficient for conjugal transfer of the element and which is located near the oriT. This same region proved to be sufficient for mobilization of coresident plasmids and unlinked integrated elements as well as for self-transfer, indicating that all of these activities are mediated by the same transfer system. Previously, we had reported that disruption of a gene, rteC, abolished self-transfer of the element. rteC is one of a set of rte genes that appears to mediate tetracycline induction of transfer activities of the conjugative transposons. On the basis of these and other data, we had proposed that RteC activated expression of transfer genes. We have now found, however, that when the transfer region of TcrEmr DOT was cloned as a plasmid that did not contain rteC and the plasmid (pLYL72) was tested for transfer out of a Bacteroides strain that did not have a copy of rteC in the chromosome, the plasmid was self-transmissible without tetracycline induction. This and other findings suggest that RteC is not an activator transfer genes but is stimulating transfer in some other way.
许多拟杆菌临床分离株含有大型接合转座子，这些转座子从供体基因组中切除，并通过一种需要细胞间接触的过程将自身转移到受体细胞。有人提出，接合转座子的转移中间体是一个共价闭合环，它通过与接合质粒相同类型的滚环机制进行转移，但接合转座子的转移起始位点此前尚未定位和表征。我们现已鉴定出一种拟杆菌接合转座子TcrEmr DOT的转移起始位点（oriT）区域，并表明它位于接合转座子的中部附近。我们还鉴定出接合转座子的一个16kbp区域，该区域对于该元件的接合转移是必需且充分的，并且位于oriT附近。事实证明，同一区域对于共居质粒和非连锁整合元件的动员以及自我转移也是足够的，这表明所有这些活性均由同一转移系统介导。此前，我们曾报道，一个基因rteC的破坏消除了该元件的自我转移。rteC是一组rte基因之一，这些基因似乎介导接合转座子转移活性的四环素诱导。基于这些及其他数据，我们曾提出RteC激活转移基因的表达。然而，我们现在发现，当将TcrEmr DOT的转移区域克隆为不含rteC的质粒，并在染色体中没有rteC拷贝的拟杆菌菌株中测试该质粒（pLYL72）的转移时，该质粒在没有四环素诱导的情况下是自我可转移的。这一发现及其他结果表明，RteC不是转移基因的激活剂，而是以其他方式刺激转移。
