The Infective Nucleic Acid from Tobacco Mosaic Virus 305 



degradation at o-i m and with a lesser rate at o-02M concentration. However, 

 pyrophosphate produces an appreciably slower degradation than other salts, as 

 indicated by both the loss in sedimentability and in infectivity. This is of par- 

 ticular interest, because pyrophosphate has previously been found to represent 

 a particularly favourable buffer for the reconstitution of virus from protein and 

 nucleic acid [5]. With this buffer and tinder optimal conditions, which includes 

 good protein and nucleic acid preparations, activities as high as 30% to 50% of 

 the theoretical, based on the weight of nucleic acid, have often been obtained. 

 The present studies suggest that pyrophosphate is of such advantage because 

 this salt, while favouring the aggregation of protein and nucleic acid no less than 

 other salts, causes the secondary degradation of the nucleic acid to proceed at 

 a much slower rate than do other salts. Thus the nucleic acid can be built into 

 its protecting coat of protein with greater efficiency in the presence of this 

 than of other salts. 



The mechanism of the protecting effect of pyrophosphate on TMV-RNA is 

 not yet clear. Very preliminary studies with ^-ip-labelled pyrophosphate indicate 

 a definite binding of this ion to the polynucleotide chain, greatly in excess of 

 the binding of phosphate under similar conditions. Since nucleotide-diphosphates 

 have been found to act similarly to pyrophosphate in some of the experiments 

 mentioned above, it is tempting to speculate that there may be metabolic signi- 

 ficance in the binding of the pyrophosphate group by a nucleic acid endowed 

 with replication-stimulating activity. 



An early tentative interpretation of the effect of pyrophosphate and other 

 diphosphates was that these acted through their abiHty to chelate metals [5]. 

 This led to a study of the effects of metals on the infectivity of TMV-RNA. 

 And indeed, the infectivity is lost upon incubation with most metals, and par- 

 ticularly with copper and iron, of which only lO"^ m concentrations are required. 

 This inactivation is definitely time- and temperattire-dependent, and thus cannot 

 be attributed solely to an ion-interchange. The addition of pyrophosphate or 

 ethylenediamine tetraacetate largely prevents this inactivation. Metal-chelation 

 may well represent an additional benefit of the use of poyrophsphate. But its 

 primary mode of action seems to be due to direct interaction with the nucleic acid. 



I would now like to mention briefly some lines of research of our laboratory, 

 aiming at chemical modification of certain groups of the nucleic acid. Dr 

 Staehelin has studied in some detail the interaction of the nucleic acid with 

 formaldehyde, described by us a few years ago. And a parallel study is being 

 performed with iodoacetate. Both reagents contain i^c, which has enabled us 

 to detect very small amounts, and arrive at the conclusion that both reagents 

 substitute amino groups in the nucleic acid and cause inactivation when only a 

 very small proportion of these groups (about one or two in a thousand) have 

 been substituted. In these studies the availability of the various synthetic poly- 

 nucleotides, of which Professor Ochoa kindly supplied us with samples, has 

 been of greatest value. It seems probable that the chemical basis of the pre- 

 paration of formaldehyde vaccines from viruses resides in this reactivity of the 

 nucleic acid. 



This has been a survey of recent work concerned with some physico-chemical 



