Case Ryan B, Chang Yun-Pei, Smith Steven B, Gore Jeff, Cozzarelli Nicholas R, Bustamante Carlos
Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA.
Science. 2004 Jul 9;305(5681):222-7. doi: 10.1126/science.1098225. Epub 2004 Jun 3.
Condensins are conserved proteins containing SMC (structural maintenance of chromosomes) moieties that organize and compact chromosomes in an unknown mechanism essential for faithful chromosome partitioning. We show that MukBEF, the condensin in Escherichia coli, cooperatively compacts a single DNA molecule into a filament with an ordered, repetitive structure in an adenosine triphosphate (ATP) binding-dependent manner. When stretched to a tension of approximately 17 piconewtons, the filament extended in a series of repetitive transitions in a broad distribution centered on 45 nanometers. A filament so extended and held at a lower force recondensed in steps of 35 nanometers or its multiples; this cycle was repeatable even in the absence of ATP and free MukBEF. Remarkably, the pattern of transitions displayed by a given filament during the initial extension was identical in every subsequent extension. Hence, after being deformed micrometers in length, each filament returned to its original compact structure without the addition of energy. Incubation with topoisomerase I increased the rate of recondensation and allowed the structure to extend and reform almost reversibly, indicating that supercoiled DNA is trapped in the condensed structure. We suggest a new model for how MukBEF organizes the bacterial chromosome in vivo.
凝聚素是含有SMC（染色体结构维持）部分的保守蛋白，其以一种对准确的染色体分配至关重要的未知机制来组织和压缩染色体。我们发现，大肠杆菌中的凝聚素MukBEF以依赖三磷酸腺苷（ATP）结合的方式，协同将单个DNA分子压缩成具有有序重复结构的细丝。当拉伸至约17皮牛顿的张力时，细丝在以45纳米为中心的宽分布中以一系列重复转变的方式伸展。如此伸展并以较低力保持的细丝以35纳米或其倍数的步长重新凝聚；即使在没有ATP和游离MukBEF的情况下，这个循环也是可重复的。值得注意的是，给定细丝在初始伸展过程中显示的转变模式在每次后续伸展中都是相同的。因此，在长度变形微米后，每条细丝在不添加能量的情况下恢复到其原始紧密结构。用拓扑异构酶I孵育可提高重新凝聚的速率，并使结构几乎可逆地伸展和重新形成，这表明超螺旋DNA被困在凝聚结构中。我们提出了一个关于MukBEF如何在体内组织细菌染色体的新模型。
