Lanka E, Wilkins B M
Max-Planck-Institut für Molekulare Genetik, Abteilung Schuster, Berlin, Federal Republic of Germany.
Annu Rev Biochem. 1995;64:141-69. doi: 10.1146/annurev.bi.64.070195.001041.
Bacterial conjugation is an important source of genetic plasticity. The initiation complex for conjugative transfer of transmissible plasmids--the relaxosome--is a specific DNA-protein structure that has been isolated from cells and reconstituted from purified components in vitro. Complexes containing uncleaved DNA and DNA cleaved at the nicsite in the origin of transfer (oriT) coexist in equilibrium. Relaxase is usually loaded onto oriT by accessory DNA-binding proteins. Relaxase catalyzes cleavage of a specific phosphodiester bond at nic and becomes covalently linked through a tyrosyl residue to the 5' terminus of the cleaved strand. Cleaved DNA may be unwound for transfer by a plasmid-encoded helicase. Single-strand transfer is thought to occur by a replicative rolling circle mechanism. Termination of a round of transfer is achieved by the cleaving-joining activity of the relaxase linked to the 5' end of the transferring strand. Relationships between DNA processing reactions and conjugative interactions of cell envelopes are particularly obscure aspects of the conjugation cycle.
细菌接合是遗传可塑性的重要来源。可转移质粒接合转移的起始复合物——松弛体——是一种特定的DNA-蛋白质结构，已从细胞中分离出来，并在体外由纯化的组分重构而成。含有未切割DNA和在转移起始点（oriT）的切口位点处切割的DNA的复合物以平衡状态共存。松弛酶通常由辅助DNA结合蛋白加载到oriT上。松弛酶催化在切口处特定磷酸二酯键的切割，并通过酪氨酸残基与切割链的5'末端共价连接。切割后的DNA可能会被质粒编码的解旋酶解开以便转移。单链转移被认为是通过复制型滚环机制发生的。一轮转移的终止是通过与转移链5'末端相连的松弛酶的切割-连接活性实现的。DNA加工反应与细胞包膜的接合相互作用之间的关系是接合循环中特别模糊的方面。
