114 R. MARKHAM 



A. Purification 



The sap of infected tobacco plants contains about 100 nig. /ml. of the virus, 

 and it may be clarified by the addition of K2HPO4, or better by heating 

 to 55°C. The virus may then be precipitated by 0.25 saturation with ammon- 

 ium sulfate (Bawden and Pirie, 1938a; Bawden and Kleczkowski, 1948) or 

 by adjustment of the pH to 4.5 or just below. It may also be concentrated 

 by high-speed centrifugation (Loring and Wyckoff, 1937; Loring, 1938). 

 The sedimentation coefficient is about 124 S. (Lauffer and Cartwright, 

 1952). 



The virus analyzes for a nucleoprotein with about 5 % of nucleic acid 

 (Bawden and Pirie, 1938a) but contains an excess of carbohydrate. The 

 quantity of the latter is variable. Some 75 % of the ribonucleic acid associ- 

 ated with the virus may be prepared by the (somewhat drastic) method 

 of Johnson and Harkins (1929) and the denatured protein stiU has 1.5-2.5 % 

 of carbohydrate associated with it. 



The virus is precipitated by 50 % (v. /v.) ethanol and denatured by 85 % 

 ethanol, and also by heating to 66°C., or by sodium dodecyl sulfate. It is 

 precipitated from salt-free solution by pancreatic ribonuclease, which, 

 however, does not digest the nucleic acid until the virus is denatured (Bawden 

 and Kleczkowski, 1948). On the other hand, the virus is readily digested by 

 trypsin, or, even more readily, by chymotrypsin. This interesting character- 

 istic has not been exploited so far, but Knight (1955a) has investigated the 

 action of carboxypeptidase, and has found that the major amino acid 

 liberated is alanine (as in cucumber viruses 3 and 4), but a fairly large 

 number of other amino acids are also released at the same time. 



One of the more characteristic properties of this virus is its tendency to 

 form jellies. The virus particles themselves are long flexuous threads, about 

 500 m/A in length (Bode and Paul, 1955) and are thinner than are tobacco 

 mosaic virus rods. Being flexible, they entwine, forming ropelike structures, 

 and, particularly if they are treated with salt solutions, they form these 

 jeUies. This characteristic is one which has, no doubt, prevented potato X 

 virus from being as extensively studied as has the tobacco mosaic virus, 

 but is not at all a disadvantage as far as routine chemical examination is 

 concerned. The precipitated, aggregated virus may be rendered soluble to 

 a large extent by the use of trypsin, with the loss of a fraction of its infectivity, 

 and it would be fascinating to see this followed by electron microscopy and 

 some of the other recent methods. 



The nucleic acids of a number of strains have been analyzed (Markham, 

 1953a; Knight, 1954) but the proteins of this interesting group of viruses 

 have received very little attention. This is all the more surprising, because 

 the immunological relationships of this group of viruses have been studied 

 by several workers, and on the whole the group is even more diverse than 



