314 H. K. SCHACHMAN AND R. C. WILLIAMS 



revealing fibers that occasionally extended as far as 3 ju, from the degraded 

 end of the virus (Fig. 9). A rough proportionality was found between the 

 maximum lengths of RNA fibres and the amount of virus rod that was 

 degraded, based upon the assumption that most of the rods were initially 

 3000A long. The value of the proportionality constant was such as to lead 

 Hart to conclude that the uncoiled RNA fibres are probably single poly- 

 nucleotide strands, possibly contaminated with small amounts of residual 

 protem. No decision could be reached as to whether a single RNA strand 

 follows the pitch of the helix, or whether there is a single strand which is 

 folded back and forth parallel to the axis of the virus rod. 



As has been mentioned previously, tobacco mosaic virus may be degraded 

 by several methods to provide a separation of its protein and RNA con- 

 stituents (Bawden and Pirie, 1940a,b; Lauffer and Price, 1940; Lauffer and 

 Stanley, 1943; Lauffer, 1943b; Schramm, 1947). Degradation of the virus in 

 mildly alkaline solutions (pH about 9.5) produces a relatively homogeneous 

 protein fragment, the "A" protein of Schramm (1947). This protein, having 

 a molecular weight of about 10^, tends to aggregate to form long rodlike 

 particles morphologically similar to those of TMV (except for a much broader 

 length distribution), and it may be "coaggregated" with purified RNA from 

 TMV to form reconstituted, infective virus (Fraenkel-Conrat and Williams, 

 1955). During the degradation of the virus several discrete, sharp boundaries 

 were observed in the ultracentrifuge, leading Schramm (1947) to propose 

 that the boundaries corresponded to different subunits of the virus. Con- 

 tinuation of this work by Harrington and Schachman (1956) showed that 

 many of the presumed degradation products were, in fact, the result of 

 aggregation of smaller products which formed and then quickly aggregated. 

 From a kinetic study of the formation and disappearance of the different 

 components these workers concluded that the degradation process involved 

 the "stripping-off" or unraveling of protein units from only one end of the 

 particles. This stripping off is rapid mitil about two-thirds of the protein is 

 removed, leading to a fragment having one-third the length of intact virus. 

 Additional evidence for a structural polarity in TMV rods is provided by the 

 micrographs of partially degraded virus (Hart, 1956). Only very rarely was 

 the central core detectable at both ends of the same particle. Instead, the 

 core was exposed at one end only. Both the protein (A-protein) and the one- 

 third fragment aggregate to larger particles. The former gives inhomogeneous 

 material of molecular weight about 10®; they are almost certainly the short 

 disklike particles seen in electron micrographs of partially degraded virus 

 (Fig. 10). They appear to have a height about 30-50A and a diameter of 150A 

 with a central hole of 60A diameter. This detection in electron micrographs 

 leads to the impression that these doughnut-like objects are removed as such 

 from aromid the RNA core, but this interpretation is eliminated by the 



