Skordalakes Emmanuel, Berger James M
Department of Molecular and Cell Biology, University of California, Berkeley, 239 Hildebrand Hall, #3206, Berkeley, CA 94720, USA.
Cell. 2003 Jul 11;114(1):135-46. doi: 10.1016/s0092-8674(03)00512-9.
In bacteria, one of the major transcriptional termination mechanisms requires a RNA/DNA helicase known as the Rho factor. We have determined two structures of Rho complexed with nucleic acid recognition site mimics in both free and nucleotide bound states to 3.0 A resolution. Both structures show that Rho forms a hexameric ring in which two RNA binding sites--a primary one responsible for target mRNA recognition and a secondary one required for mRNA translocation and unwinding--point toward the center of the ring. Rather than forming a closed ring, the Rho hexamer is split open, resembling a "lock washer" in its global architecture. The distance between subunits at the opening is sufficiently wide (12 A) to accommodate single-stranded RNA. This open configuration most likely resembles a state poised to load onto mRNA and suggests how related ring-shaped enzymes may be breached to bind nucleic acids.
在细菌中，主要的转录终止机制之一需要一种名为Rho因子的RNA/DNA解旋酶。我们已经确定了Rho与核酸识别位点模拟物在游离状态和核苷酸结合状态下的两种结构，分辨率达到3.0埃。两种结构均显示，Rho形成一个六聚体环，其中有两个RNA结合位点——一个主要位点负责识别靶标mRNA，另一个次要位点是mRNA转运和解旋所必需的——指向环的中心。Rho六聚体并非形成一个封闭的环，而是裂开的，其整体结构类似于一个“锁紧垫圈”。开口处亚基之间的距离足够宽（12埃），以容纳单链RNA。这种开放构型很可能类似于一种准备加载到mRNA上的状态，并提示了相关的环形酶可能如何打开以结合核酸。
