Determinants of Conjugative Transposition

接合转座子从一个细菌宿主移动到另一个细菌宿主需要从供体染色体切除转座子，将切除的转座子从供体接合转移到受体，并将转座子整合到受体染色体中。研究最多的接合转座子是Tn916。Tn916的移动由转座子编码的二价Int蛋白催化，其结构域识别并结合两个不同的DNA序列。 Tn916不移动相邻的染色体遗传标记，并且参与接合转移的基因的表达依赖于转座子的切除和环化。因此，Tn916接合转座的第一步是切除该元件，并且接合转移在切除之前不会发生。第二个转座子编码的蛋白质是切除所需的，Xis，在转座子切除中起着调节作用，最近提出了Xis功能的模型。将对该模型进行测试。转座子切除和转移可以通过从异源启动子表达Int和Xis来解离。 转座子切除在供体细胞群体中以大大提高的频率发生，但接合转移和整合的频率在这些条件下保持不变。这意味着配偶转移的步骤也可能受到管制。Int重组酶除了结合和切割转座子末端之外，还显示出特异性结合至接合DNA转移的起点oriT。将确定Tn916的配偶转移是否需要Int，或者Int是否在该过程中起调节作用。对Int与oriT结合的研究表明，Int的N端DNA结合结构域可能对C端催化结构域的活性产生影响。Int的两个DNA结合结构域之间的这种相互作用的可能作用将被检查。接合转座子，如Tn916具有显着的特性，不仅能够从一个单一的细菌物种的基因组中的位置移动到另一个位置，但令人惊讶的是，大量的不同的细菌物种之间的转移。它们也有能力调动通常不能自我传播的其他遗传因素。 这些特性意味着接合转座子在促进自然环境中复杂微生物群落的遗传交换方面可能具有重要意义。这些复杂的群落对人类活动有着巨大的影响。对这些群落中遗传交换因子的研究将增加我们对微生物种群如何适应不断变化的条件并繁荣发展的理解。

Movement of conjugative transposons from one bacterial host to another requires transposon excision from the donor chromosome, conjugal transfer of the excised transposon from donor to recipient and integration of the transposon into the recipient chromosome. The best-studied conjugative transposon is Tn916. Movement of Tn916 is catalyzed by the transposon-encoded bivalent Int protein, whose domains recognize and bind to two distinct DNA sequences. Tn916 does not mobilize adjacent chromosomal genetic markers and expression of the genes involved in conjugal transfer is dependent upon excision and circularization of the transposon. Thus, the first step in conjugative transposition of Tn916 is excision of the element, and conjugal transfer does not occur prior to excision. A second transposon encoded protein that is required for excision, Xis, plays a regulatory role in transposon excision and a model for Xis function has recently been suggested. This model will be tested. Transposon excision and transfer can be dissociated by expression of Int and Xis from a heterologous promoter. Transposon excision occurs at a greatly elevated frequency in the donor cell population but the frequency of conjugative transfer and integration remain unchanged under these conditions. This implies that the step of conjugal transfer may also be regulated. The Int recombinase, in addition to its binding and cleavage of the transposon ends, has been shown to bind specifically to the origin of conjugal DNA transfer, oriT. Whether Int is required for conjugal transfer of Tn916, or if Int plays a regulatory role in this process will be determined. Examinion of Int binding to oriT has suggested that the N-terminal DNA binding domain of Int may exert an effect on the activities of the C-terminal catalytic domain. The possible role of such interactions between the two DNA binding domains of Int will be examined.Conjugative transposons such as Tn916 have the remarkable property of being not only able to move from place to place in the genome of a single bacterial species, but to transfer themselves between an amazingly large number of diverse bacterial species. They also have the ability to mobilize other genetic elements that normally are not self-transmissible. These properties mean that conjugative transposons are likely to be of major importance in promoting genetic exchange in the complex microbial communities that exist in natural environments. These complex communities have a great impact on human activities. The study of agents of genetic exchange in these communities will increase our understanding of how microbial populations adapt to changing conditions and prosper.