THE BIOCHEMISTRY OF PLANT VIRUSES 59 



to Lipkin and Talbert (1955). In this method, which has not yet been fully 

 exploited, the whole virus may be dried, dissolved in anhydrous formamide, 

 and then exposed to the action of potassium methoxide. After the reaction 

 is complete, the reactants are removed, still in the absence of water; the 

 resultant mixture contains nucleotide methyl esters, and any nucleotides 

 which are liberated as such must have originated as chain termini. The 

 method is exceedingly exacting, but, in the presence of internal dehydrating 

 agents, should give quite unequivocal results. 



The possible kinds of linear polynucleotides are shown diagrammatically 

 in Table III, which shows a chain of 4 (arbitrary) nucleotides having the 



TABLE III 



Types of Polynucleotide Chain a 



Type Polynucleotide chain 



1 A — ^p — B — p — C — p — ^D — p 



2 A— p— B— p— C— p— D 



3 p— A— p— B— p— C— p— D 



4 p — A — p — B — p — C — p — D— p 



5 A— p— B— p— C— p— D— p! 



6 p— A— p— B— r»— C— p— D— p! 



* A, B, C and T> are nucleoside residues. The phosphate residues joining 3' - 5' in 

 adjacent nucleoside residues are denoted by p, 3' - 5' being from left to right. The 

 exclamation mark denotes a 2' : 3' cylic phosphate residue. 



6 different kinds of ends that are possible. Of these, type 1 can have its 

 right hand end detected after dephosphorylation, or if C happens to be a 

 pyrimidine nucleoside and D a purine nucleoside. Type 2 is type 1 dephos- 

 phorylated (or types 3 and 4 dephosphorylated), and the terminal D residue 

 is liberated by alkaline hydrolysis. On alkaline hydrolysis, type 3 gives 

 p — A — p, which is a nucleoside diphosphate, and D, which is a nucleoside. 

 Under the same conditions type 4 will give p — A — p as a detectable end, 

 as will type 6. Type 5 can be treated with ribonuclease if C is a pyrimidine 

 nucleoside and D a purine nucleoside, when D — p! is liberated. Actually, 

 cyclic phosphate ends may be hydrolyzed specifically if necessary, and the 

 chains 5 and 6 converted to types 1 and 4. Chain ends detectable on methan- 

 olysis are the right ends of 1, 2, 3, and 4 and the left ends of 3, 4, and 6, 

 which would become monomethyl esters of p— A — p. 



At one time it was thought that the tobacco mosaic virus had ends of 

 the type 3 (Markham et al., 1954). These ends were certainly detected, but 

 were probably representative of an external small polynucleotide contamin- 

 ant. The relative amounts of the ends were relatively large, being about 

 1 to every 50 residues. If, as is now thought, the nucleic acid cham is con- 

 tinuous and single, the number of ends would be about 1 in every 8000, a 



