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

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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.