448 H. FRAENKEL-CONRAT 



preparations show the same decreasing order in their sedimentabihty in the 

 presence of the same salts, the conclusion is strongly suggested that the salt 

 causes a specific base-paired aggregation in nucleic acids of appropriate 

 structure, and that this leads to an increase in molecidar or particle weight 

 in the solution, and possibly to the production of infectious nucleic acid. 



Another series of observations in support of this concept is described in 

 detail in a subsequent section. It consists of a study of the nature of virus 

 reconstituted from a mixture of two nucleic acids. Only if subunits exist in 

 solution would one expect a definite effect of one type of nucleic acid on 

 another, whereas complete viral nucleic acid complements should not be 

 greatly affected by the presence of another nucleic acid. The finding of evi- 

 dence for definite interactions between two types, if mixed prior to addition 

 to protein and buffer, seems again to require the existence of active subunits. 



One approach to the problem of the molecular weight of an unbranched 

 chain polymer, such as RNA is now regarded, would appear to be an analysis 

 of the number of end groups. This is complicated in nucleic acids by the 

 potentially varied nature of the terminal residues, which can be 5' phos- 

 phates, 3' phosphates, 2'-3' cyclic phosphates, or unphosphorylated terminal 

 residues from either end of the chain. Earlier claims that there were many 

 5' phosphate end groups in TMV have been corrected (Reddi and Knight, 

 1957) and withdrawn (Matthews and Smith, 1957). At present there is little 

 positive evidence for the occurrence of measurable amounts of any end 

 groups of the various types listed. However, if one considers that the ex- 

 pected chain lengths are within the range of 1000 to 10,000 units long, the 

 technical difficulties in detecting the 0. 1 % or fewer nucleotides which might 

 be terminal loom large. 



B. Lability 



The fact that TMV-RNA degrades upon storage has been known since its 

 first description by Cohen and Stanley (1942). Hopkins and Sinsheimer 

 (1955) made similar observations with higher molecular weight preparations. 

 Since the discovery of the infective activity of RNA, this activity was found 

 a useful criterion in the study of the instabihty of RNA. According to Gierer 

 and Schramm, only freshly prepared nucleic acid is fully infectious and of 

 high molecular weight. With the SDS-type of preparations, the infectivity 

 was generally found to be stable during many months of storage at — 60°C.; 

 also at 0°C., many preparations decreased only slowly in infectivity. It then 

 became evident that the difference in stability in the two types of prepara- 

 tions was due to the presence of 0.02 M phosphate in the phenol type of pre- 

 paration, and the virtual absence of salts from the SDS-type of preparation. 

 Most preparations of either type were found to degrade rapidly in the presence 



