THE BIOCHEMISTKY OF PLANT VIRUSES 81 



consist of particles having a sedimentation coefficient of about 4.6 /S and a 

 molecular weight of about 90,000, i.e., they represent about 1/500 of the 

 protein of the virus, and as we shall see must be made up from some 6 (or 

 3) subunits. The A-protein is only really stable in a dispersed form in alkaline 

 solution; on acidification to pH 6 it begins to aggregate and forms short 

 cylinders having a hollow axis. Eventually it turns into fibers which resemble 

 the virus in shape, but which have no definite length. They also have an 

 X-ray diffraction pattern very much like the intact virus (Franklin, 1955b). 

 The rods are, however, very unstable to heat compared with the whole virus 

 (Schramm and Zillig, 1955), and dissolve very much more readily in alkaline 

 solutions. In fact the polymer of the A-proteiti is only really stable over the 

 pH range 2.5-6, Over this range, the mobility on electrophoresis is the same 

 as that of the intact virus, presumably because the surface charge distribu- 

 tions are the same, even though the total number of charged groups in the 

 molecules is different. This effect has been shown for at least three strains 

 of the virus (Kramer and Wittmann, 1958). 



Outside the zone of stability of the polymer the mobility changes, presum- 

 ably because of the breakdown of the rods and the subsequent exposure of 

 other charged groupings which had been hidden previously. This is of course 

 only one of the instances where it has been shown that the nucleic acid of 

 plant viruses does not appear to contribute to their electrophoretic mobility. 



In the presence of nucleic acids and even of synthetic polynucleotides and 

 partly degraded nucleic acid the A-protein may be polymerized in such a 

 way that it actually includes the former (Hart and Smith, 1956), and gives 

 rise to rods which resemble virus in their greater stability. In fact, if the 

 polymerization is carried out in the presence of carefully prepared virus 

 nucleic acid (Fraenkel-Conrat and Williams, 1955), not necessarily even from 

 the same strain of virus (Fraenkel-Conrat and Singer, 1957), the so-caUed 

 "reconstitution" ensues when a small proportion of material having infectivity 

 is formed. When a mixed reconstitution is performed, the progeny is decided 

 by the donor of the nucleic acid component. This phenomenon is discussed 

 elsewhere. 



H. The X-Protein 



Infected plants themselves contain a protein (or proteins), the X-protein 

 of Takahashi and Ishii (1952a,b, 1953), Jeener and Lemoine (1953; Jeener 

 et al., 1954) which resembles the A-protein in many ways. This protein 

 was first recognized by Bawden and Pirie (1945b, 1956), who thought it was 

 a small form of the tobacco mosaic virus nucleoprotein. The methods which 

 they used for the isolation of this protein were, however, such as to cause 

 a certain amount of polymerization to take place, and so their material 



VOL. II — 6 



