X-Ray Crystal Structural Analysis of Tobacco Necrosis Virus at 5A Resolution

烟草坏死病毒（TNV）是一种直径约280 a的小球形病毒（Smith and Bald, 1934），由180个外壳蛋白拷贝和分子量约为1.3 x 10^6的单链RNA组成。与它的卫星病毒，卫星烟草坏死病毒（STNV）分离。STNV直径约200 A，由60个外壳蛋白拷贝和单链RNA组成，其晶体结构已在2.5 A分辨率下被澄清（Jones and Liljas, 1984）。STNV只有在它的助手TNV的帮助下才能繁殖；因此，从病毒学的角度来看，TNV和STNV之间的功能和结构关系是重要的。利用光子工厂的同步辐射，利用半径430 mm的大卡带Weissenberg照相机，以5 A分辨率获得了烟草坏死病毒（TNV）的x射线强度数据。病毒的二十面体对称性以及病毒颗粒在单位细胞内的包装均通过旋转函数进行了详细研究。假设亚基为球形，其在二十面体表面的排列由30 A分辨率的r因子搜索确定。该病毒在亚基排列上与南方豆花叶病毒相似。用分子置换平均法测定晶体结构。从8A分辨率图开始，经过100次左右的细化，得到了5A分辨率的电子密度图。实测结构因子和计算结构因子的最终r因子和相关系数分别为0.16和0.92。目前正在利用地图建立TNV模型。

Tobacco Necrosis Virus (TNV) is a small spherical virus about 280 A in diameter (Smith and Bald, 1934), consisting of 180 copies of coat protein and a single-stranded RNA with a molecular weight of about 1.3 x 10^6. It has been isolated with its satellite virus, Satellite Tobacco Necrosis Virus (STNV). STNV, about 200 A in diameter, consists of 60 copies of coat protein with a single-stranded RNA, and its crystal structure has been clarified at 2.5 A resolution (Jones and Liljas, 1984). STNV can multiply only with the assistance of its helper, TNV ; thus the functional and structural relationships between TNV and STNV are important from a virological viewpoint.X-ray intensity data for Tobacco Necrosis Virus (TNV) were obtained at 5 A resolution with a Weissenberg camera with a large cassette of 430 mm radius using synchrotron radiation at the Photon Factory. The icosahedral symmetry of the virus as well as packing of the virus particles in a unit cell were scrutinized by the rotation function. Assuming the subunit to be spherical, its arrangement on the icosahedral surface was determined by an R-factor search at 30 A resolution. The virus was found to be similar to southern bean mosaic virus in its subunit arrangement.Crystal structure was determined by the molecular replacement averaging. Electron density map at 5A resolution was obtained after about 100 cycles of the refinement starting at 8A resolution map. The final R-factor and correlation coefficient for the observed and calculated structure factors were 0.16 and 0.92, respectively. Model building of TNV is being carried out by using the map.