RNA as Genetic Material 



365 



figure 28-2. Amino acid sequence in the protein building block of tobacco 

 mosaic virus (TMV). There are 158 amino acids in the sub-unit, the encircled 

 residues indicate the points of digestion by trypsin. (Courtesy of A. Tsugita, 

 D. T. Gish, J. Young, H. Fraenkel-Conrat, C. A. Knight, and W . M. Stanley, 

 Proc. Nat. Acad. ScL, U.S., 46:1465, 1960.) 



therefore, that pure virus RNA is infective 

 and carries all the genetic information neces- 

 sary for its replication.* These experiments 

 also prove that TMV protein plays no part — 

 other than protecting the RNA and increas- 

 ing the infectivity — in the replication either 

 of the RNA genetic material or of itself. 

 This conclusion is tested by what is called 

 reconstitution experiments in which, under 

 appropriate conditions, we can first separate 

 the protein and RNA of TMV and then have 

 them recombine and demonstrate the high 

 infectiveness of the original virus. Using 

 two genetically-different strains of this virus 

 — the standard (TMV) and Holmes rib 

 grass (HR) — a highly-infective virus con- 



4 RNA isolated from a number of small animal 

 viruses and from coliphages f2, MS2, etc., is also 

 infective. (See Vol. 27, Cold Spring Harb. Sym- 

 pos. Quant. Biol., 1962, and D. E. Engelhardt and 

 N. D. Zinder, 1964.) 



taining the RNA of TMV and the protein 

 coat of HR can be constructed. The prog- 

 eny obtained are typically TMV with both 

 TMV RNA and protein coat. The recipro- 

 cal construct, a virus with HR RNA and 

 TMV protein, produces HR progeny typical 

 in both RNA and protein. Thus, only the 

 RNA of a TMV particle specifies the RNA 

 and protein of the progeny virus. 5 Muta- 

 tions can be induced in TMV by many of the 

 agents known to be mutagenic for DNA. 

 Such results and others prove that the bio- 

 logical activity of the RNA depends upon its 

 primary (nucleotide content) and not its sec- 

 ondary (coiling pattern) structure. 



The complete amino acid sequence in the 

 protein building block of tobacco mosaic 



5 The genetic experiments described for TMV are 

 based largely upon work of H. Fraenkel-Conrat 

 and R. C. Williams (1955), A. Gierer (1960). 

 G. Schramm, and others. 



