THE STRUCTURE OF VIRUSES 



129 



obtained in crystalline form to date. Ber- 

 nal and Fankuclien made X-ray diffraction 

 studies on oriented crystals and on solu- 

 tions of virus preparations and found no 

 difference between strains of latent-mosaic 

 virus, and only doubtful ones in the case 

 of the cucumber-mosaic-3 and -4 viruses. 

 However, they found definite differences in 

 intensity of the intermolecular pattern in 

 the case of the strains of tobacco-mosaic vi- 

 rus, although the intramolecular pattern 

 appeared to be the same. In every case 

 the intramolecular pattern was found to be 

 independent of the concentration of the 

 virus while the intermolecular pattern 

 varied continuously and quantitatively 

 with the concentration. The intramolecu- 

 lar pattern of the bushy-stunt virus was 

 found to be of essentially the same type as 

 that of tobacco-mosaic virus, indicative of 

 a repeat-unit of approximately 20 x 20 x22 

 A. Bernal came to the interesting conclu- 

 sion that the individual molecules of to- 

 bacco-mosaic virus have an internal crys- 

 talline structure, that this structure is 

 analogous to that of other crystalline pro- 

 teins, and hence that each molecule may be 

 regarded as a crystal. Although the beauti- 

 ful dodecahedric crystals of bushy-stunt vi- 

 rus are regarded as true crystals, Bernal 

 believes that the intermolecular pattern of 

 crystalline tobacco-mosaic virus indicates 

 the presence of long molecules arranged 

 with a perfect hexagonal, 2-dimensional 

 regularity at right angles to the length but 

 with no regularity in the direction of the 

 length. He is of the opinion that the solid 

 needle-shaped particles which are visible 

 under the microscope and which have been 

 referred to as crystals have only this type 

 of regularity and are therefore really in a 

 liquid crystalline state and should be re- 

 ferred to as liquid or para-crystals. 

 Wyekoff and Corey have also studied the 

 X-ray diffraction pattern of crystalline 

 tobacco-mosaic virus, and although they ob- 

 tained essentially the same pattern as that 

 reported later by Bernal, they interpreted 

 it as resulting from true crystals. 



Recently, Kausche reported that he had 

 succeeded in preparing in vitro the hexa- 



gonal crystals of tobacco-mosaic virus 

 which previously had been noted only 

 within living cells. Beale had previously 

 described the transformation of the hexag- 

 onal crystals within living cells into the 

 needle crystals upon the addition of acid. 

 Kausche reported that he had observed 

 under the microscope the reversal of this 

 phenomenon, namely, that bundles of the 

 needle crystals fused together to form the 

 hexagonal crystals. He considers that the 

 molecules, the short and long fibers or fila- 

 ments described by Best, the needle crys- 

 tals, and the hexagonal crystals form an 

 unbroken series. The preparation of the 

 hexagonal crystals in vitro should make it 

 possible to determine whether or not they 

 are more complex in composition than the 

 needle crystals, a possibility which was 

 suggested by Beale. Bernal has stated that 

 the hexagonal crystals occurring within 

 cells ''possess end as well as side faces and 

 undeniably show 3-dimensional regularity ' ' 

 and Bawden also considers them to be true 

 crystals. However, Bernal and Bawden 

 consider that the needle crystals are not 

 true crystals and possess only a 2-dimen- 

 sional regularity; hence, if the hexagonal 

 crystals are formed from the needle crys- 

 tals, a rather unusual realignment of the 

 molecules must occur. It appears prefer- 

 able to leave the question of the kind of 

 crystallinity open for the present. Whether 

 crystalline tobacco-mosaic virus possesses a 

 2- or 3-diniensional tyipe of regularity is 

 not important from the standpoint of the 

 virus w^orker, even though it may be quite 

 important to the crystallographer, for, as 

 mentioned above, crystallinity is not a vital 

 issue with respect to either the purity or 

 the nature of viruses, and regardless of the 

 final decision the general virus problem will 

 remain unchanged. 



In most of the work on the estimation of 

 the sizes of viruses, it has been tacitly as- 

 sumed that the virus particles were essenti- 

 ally spherical in shape. Recently, however, 

 Lauffer and I demonstrated that some vi- 

 ruses are very asymmetrical in shape. The 

 earliest indication of the asymmetry of a 

 virus was obtained by Takahashi and 



