Determinants of Conjugative Transposition

共轭转座子是一种从供体细菌基因组中的一个位置转位到受体细菌基因组中的新位置的遗传元件。从供体到受体的移植需要细胞间接触。它们有一个非常广泛的宿主范围，能够在不同种类和属的细菌之间转移。这些转座子中研究得最好的是Tn916和它的近亲Tn1545，它们的转座子通过一种机制转座，这种机制涉及到在供体细菌中切除转座子以形成一个圆形的中间物。转座子的这种循环形式随后转移到受体，在那里发生整合。与大多数转座因子不同，Tn916的转座不会导致目标序列的重复。相反，供体转座子两侧的两个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，要么恢复原来的目标序列，要么目标的6个碱基对被转座子引入的耦合序列所取代。两种转座子编码的蛋白质，称为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，转座子切除是夫妇转位的限速步骤。有两个因素影响共轭换位的频率。这些因素是转座子两侧的偶联序列的性质和转座子编码的辅助蛋白的存在。像Tn916这样的共轭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起作用，然而如何确定切除的频率，从而确定转座，仍然知之甚少。最近建立的转座子切除体外实验意味着偶联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验证。目前尚不清楚辅助蛋白是否在确定共轭转位频率中起重要作用。这项研究将消除主要的不确定性，这些元素是许多重要细菌物种之间基因转移的重要介质，并导致对共轭转位如何发生的更深入的了解。

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.