VAKIATION AND ITS CHEMICAL CORRELATES 141 



Fig. 1 were obtained by procedures tbougbt to yield direct series of mutants 

 (Black and Knight, 1953). The differences in biological properties of these 

 strains are not limited to the quahty of the symptoms they ehcit, for some 

 show striking differences in the severity of the disease they cause. This is 

 illustrated in Fig. 2, which contrasts the course of infection with ordinary 

 TMV with that resultmg from the strain J14D1. Purified preparations of 

 these biologically diverse strains constitute the materials for chemical 

 investigation. 



The primary operation in protein chemistry is to determine the kinds and 

 amoimts of amino acids present. Such analyses are important, not only for 

 the hght they shed on the nature of the protein concerned, but also because 

 such information is basic for subsequent structural studies. 



The first complete amino acid analyses on viruses were made on the 6 

 distinctive strains of TMV that cause the symptoms illustrated in the top 

 half of Fig. 1 (Knight, 1947). Later, 7 other strains, whose symptoms are 

 shown in the bottom half of Fig. 1, were also analyzed (Black and Knight, 

 1953). Microbiological assays were used except for cysteine, and, in one 

 series, arginine and tryptophan, for which colorimetric methods were em- 

 ployed. The results of the analyses made on the above 13 strains of TMV are 

 summarized in Table III, in which values for the strains are given only in 

 the cases that are considered to differ significantly from TMV. The values 

 shown were obtained by many analyses on different preparations of the 

 strains. An arbitrary level of significance of 10 % was chosen; in general, no 

 strain was considered to have a significantly different amino acid content 

 unless it differed from TMV by this much or more. Such a margin of error is 

 considerably greater than the mean deviation of single analyses from the 

 average and hence the estimate of significant differences is conservative. 

 Supporting evidence for the validity of this evaluation has come from sub- 

 sequent studies in which isoelectric points, peptide analysis, and other char- 

 acteristics of the preparations have proved consistent with the results re- 

 ported for the amino acid analyses (Oster, 1951; Knight, 1957). However, a 

 difficulty was encountered with the microbiological assay in that in an 

 interval of 5 years between two major groups of analyses, a shift in level of 

 values for some of the amino acids was observed. This was due to changes in 

 the microorganism used for assay rather than in the preparations, since the 

 same answer was obtained with all preparations from the current ones to 

 those 10-15 years old. For this reason, two sets of values are given for TMV 

 in Table III, one for each of the two major periods of analysis. From these 

 data, it can be concluded that some strains of T]\IV do and some do not 

 differ demonstrably in protein composition. 



b. Strains of Tomato Bushy Stunt Virus. For many years bushy stunt virus 

 (BSV) was thought to be an exception to the rule that plant viruses mutate. 



