Wentzell L M, Nobbs T J, Halford S E
Department of Biochemistry, University of Bristol, U.K.
J Mol Biol. 1995 May 5;248(3):581-95. doi: 10.1006/jmbi.1995.0244.
The SfiI endonuclease cleaves DNA by a mechanism that differs from other restriction enzymes. While most restriction enzymes are dimeric proteins that interact with a single DNA site, SfiI exists in solution as a tetramer and it appears to interact with two copies of its recognition sequence before it can cleave DNA. Its primary reaction is then to cut both strands at both sites in a concerted process. The two sites can be on either the same or different DNA molecules, so SfiI provides a test system for long-range interactions on DNA. On either supercoiled or linear DNA with two sites separated by 1 kb, the bridging interaction between the sites is an intramolecular event: the majority of the DNA is converted directly to products cleaved at both sites, bypassing intermediates cut at one site. Sites on separate DNA molecules, or two sites on linear DNA several kb apart, engage in an intermolecular interaction prior to cleavage. The interaction between two DNA molecules with one site on each is impeded by supercoiling in both partners but is permitted when one partner is linear: it may require reptation of one DNA through another. SfiI reactions have marked similarities to some of the reactions catalysed by site-specific recombination enzymes.
SfiI核酸内切酶切割DNA的机制与其他限制酶不同。大多数限制酶是与单个DNA位点相互作用的二聚体蛋白，而SfiI在溶液中以四聚体形式存在，并且在切割DNA之前似乎与两个识别序列拷贝相互作用。其主要反应是在一个协同过程中在两个位点切割两条链。这两个位点可以在同一DNA分子上，也可以在不同的DNA分子上，因此SfiI为DNA上的长程相互作用提供了一个测试系统。在超螺旋或线性DNA上，两个位点相隔1 kb，位点之间的桥连相互作用是分子内事件：大多数DNA直接转化为在两个位点都被切割的产物，绕过在一个位点被切割的中间体。在单独的DNA分子上的位点，或线性DNA上相隔数kb的两个位点，在切割之前会发生分子间相互作用。两个DNA分子上各有一个位点，它们之间的相互作用在两个分子都超螺旋时会受到阻碍，但当其中一个分子是线性时则允许发生：这可能需要一个DNA分子穿过另一个分子进行爬行。SfiI反应与位点特异性重组酶催化的一些反应有明显的相似之处。
