smith: directed mutation 427 



ably the binding with protein. Attachment of DNA molecules longi- 

 tudinally as well as DNA strands laterally may be through bonding of 

 diester phosphates with a divalent metal (62). These sites, as well as 

 attachment points of DNA through diester phosphate groups to 

 amino groups of proteins, may be susceptible to attack by alkylating 

 agents which are known specifically to react with phosphate groups 

 of DNA (54). What relation may exist between such reactions and 

 DNA genetic coding is unknown, but conceivably alteration of base 

 sequence between DNA "species" could be as significant as that within 

 a single DNA molecule. 



In order to formulate a direct analogy between current explana- 

 tions for inter-allelic specificity for reverse (restoration of function) 

 mutations in microorganisms in terms of DNA base sequence and 

 inter-locus specificity for forward (loss of function) mutations in 

 higher forms, it would appear necessary to postulate a qualitative or 

 quantitative difference in reaction sites affecting DNA code within 

 the functional gene locus involved. That is, either that the gene loci 

 differ qualitatively in susceptibility to change into viable or lethal 

 mutant sequences by the action of one mutagen compared to another, 

 or that there are different numbers of ways (numerically more or 

 fewer susceptible sites) among gene loci to change to heritable loss of 

 function with one mutagen compared to another. Parenthetically, to 

 those who work with cultivated plants forward and reverse mutations 

 may often lack evident distinction. 



Close analogues of normal DNA bases have not, to date, been 

 reported as mutagenic in higher plants and animals (nebularine may, 

 however, be considered in this category). Owing to interest in the use 

 of these analogues in cancer chemotherapy much is known about their 

 incorporation and antimetabolic activity in mammalian cells (30). 

 Less is known about their effects on plants (52). We have 

 recently obtained preliminary evidence from experiments with 

 tritiated 2-aminopurine that this analogue is incorporated 

 m the DNA of nuclei of root tip cells of Vicia faba (Figure 

 1). Root tips were placed in a solution of tritiated 2-amino- 

 purine (31 X 10"° M, 3.7 [.ic/ml specific activity) for 8 hours 

 at 23° C. They were then grown in Hoagland's solution at 20° C for 

 24- and 48-hour periods, respectively; placed for 2 hours in 0.05 per 

 cent colchicine; fixed; and stained by the standard Feulgen smear 



