X. TMV STUDIES IN GENETIC CODING 493 



rubbing with an abrasive evoke lesions which then are not due to the 

 strain used in the inoculum, obviously a serious source of error. 



For the quantitative testing for infective virus particles solutions 

 containing on the order of 1 to 10 X lO"*^ mg virus/ml are rubbed onto 

 half-leaves of the Xanthi, nc variety, the most sensitive test plant, and 

 the resultant lesion number compared to that given by a known virus 

 solution, preferably applied to the opposite half-leaves of the same 

 plants. Over the range of 5 to 50 lesions per half-leaf, the response is 

 approximately proportional to the virus concentration, but in absolute 

 terms of multiplicity plant virus infection is veiy inefficient. The fact 

 that it takes somewhere between 10^ and 10' particles to produce a lesion 

 or to initiate a systemic disease has been cause for some concern as to 

 whether the population of virus particles is uniformly infective. How- 

 ever, most workers in the field tend to attribute this inefficiency of plant 

 virus infection to the crude mechanics of the wounding and infecting 

 process. The fact that most of the virus which is applied and rubbed 

 onto the leaf can be washed off a few seconds later without decreasing 

 the resultant lesion count seems to support this interpretation. 



B. COMPOSITION OF NATURAL STRAINS 



Analytical comparisons of natural strains of TMV were initiated by 

 Knight and Stanley in 1941 and the first complete amino acid analyses 

 appeared in 1947 (Knight). The strains originally investigated, the 

 masked (M), J14D1 (JD), yellow aucuba (YA), green aucuba (GA), 

 Holmes ribgrass (HR), and cucumber viruses 3 and 4 (CV3 and CV4) 

 were obtained from field isolates from various diseased plants, and all 

 but the last two were classified as strains of TMV on the basis of 

 physicochemical, serological, and biological tests.- Later Black and 

 Knight (1953) investigated two series of more closely related TMV 

 strains, in part derived under such greenhouse conditions from one 

 another, that single-step mutations may be presumed. 



Analyses of the proteins of natural strains were also reported from 

 the Max Planck Institut fiir Biologic (Aach, 1958; Wittmann, 1960b), 

 and nucleotide analysis were obtained by Black and Knight (1953), as 

 well as by Markham and Smith (1950). 



The base analyses of the RNA of different strains have revealed no 

 differences in composition, quite in contrast to the RNA of different 



^ Cucumber viruses 3 and 4 were classed as TMV strains on the basis of physico- 

 chemical relationship. When chemical .studies (RNA composition, end groups, 

 cysteine content, etc.) .set them sharply apart from all bona fide strains of TMV, 

 Knight suggested reclassification (Knight, 1955). 



