Determinants of Conjugative Transposition

接合转座子是从供体细菌基因组中的一个位置转座到受体细菌基因组中的一个新位置的遗传元件。从供者到接受者的转移需要细胞间的接触。它们有非常广泛的宿主范围，能够在不同物种和属的细菌之间转移。这些转座子中研究得最好的Tn916及其近亲Tn1545通过在供体细菌中切除转座子形成环状中间体的机制进行转座。转座子的这种环形形式然后转移到接收者，在那里发生整合。与大多数转座元件不同，Tn916的转座不会导致靶序列的复制。相反，供体中转座子两侧的两个6个碱基对序列中的一个(称为偶联序列)与Tn916一起转移到受体。切除后，要么恢复原来的靶序列，要么用转座子引入的偶联序列取代靶的6个碱基对。两个转座子编码的蛋白质Int和Xis参与了切除和整合。INT是重组酶整合酶家族的成员。目前的证据表明，转座子切除是夫妻转座中的限速步骤。有两个因素已被证明影响接合转座的频率。这些因素是转座子两侧的偶联序列的性质，以及转座子编码的辅助蛋白的存在。像Tn916这样的接合转座子最显著的特性是它们能够在各种不同的细菌宿主中工作，但如何确定切除和转座的频率仍然知之甚少。最近建立了一种转座子切除的体外检测方法，这意味着耦合序列和辅助蛋白共同决定转座子切除频率的假设可以得到严格的检验。目前尚不清楚辅助蛋白是否在决定接合转座频率方面发挥重要作用。这项研究将消除这些元素研究中的主要不确定性，这些元素是许多重要细菌物种之间基因转移的重要中介，并导致更好地理解接合转座是如何发生的。

Conjugative transposons are genetic elements that transpose from one place in the genome of a donor bacterium to a new place in the genome of a recipient bacterium. The transfer from donor to recipient requires intercellular contact. They have an extraordinarily broad host range, being able to transfer between bacteria of different species and genera. The best studied of these transposons, Tn916, and its close relative, Tn1545, transpose by a mechanism that involves excision of the transposon in the donor bacterium to form a circular intermediate. This circular form of the transposon then transfers to the recipient, where integration occurs. Unlike most transposable elements, transposition of Tn916 does not result in duplication of the target sequence. Rather, one of two 6 base-pair sequences flanking the transposon in the donor (called coupling sequences) is transferred with Tn916 to the recipient. Upon excision, either the original target sequence is restored, or 6 base pairs of the target are replaced by the coupling sequence introduced with the transposon. Two transposon-encoded proteins called Int and Xis are involved in excision and integration. Int is member of the integrase family of recombinases. Current evidence suggests that transposon excision is the rate-limiting step in conjugal transposition. Two factors have been shown to influence the frequency of conjugative transposition. These factors are the nature of the coupling sequences flanking the transposon and the presence of a transposon-encoded accessory protein. The most remarkable property of conjugative transposons like Tn916 is their ability to operate in wide variety of different bacterial hosts, yet how the frequency of excision, and thus transposition, is determined remains poorly understood. The recent establishment of an in vitro assay for transposon excision means that the hypothesis that coupling sequences and accessory proteins together determine the frequency of transposon excision can be rigorously tested. It is not clear whether accessory proteins play a significant role in determining the frequency of conjugative transposition. This research will remove major uncertainty from the study of these elements that are important mediators of gene transfer between many important species of bacteria and lead to a much greater understanding of how conjugative transposition occurs.