V. MOLECULAR MECHANISM OF MUTATIONS 233 



effect on whole organisms (Alexander and Stacey, 1958). These authors 

 propose that polyfunctional mustards exert their lethal effect by cross- 

 linking of DNA molecules rather than merely by breaking the sugar- 

 phosphate backbone of DNA. But the latter mechanism does not seem 

 negligible since monofunctional alkylating agents also have a lethal 

 effect. 



Most alkylating agents have some mutagenic effect, whether they 

 are mono- or polyfunctional. Their mutagenic strength and specificity, 

 as compared to the lethal effect, depend on the structure of the alkyl 

 group. Table II gives a list of mutagenic alkylating agents. Since they 

 have mostly been tested on whole cells or organisms it is not known 

 whether they act directly on DNA or the chromosomes, or whether their 

 effect is indirect. In contrast to the necessary fixation of UV-induced 

 mutations that requires protein synthesis no such fixation seems neces- 

 sary for the mutagenic effect of at least certain alkylating agents; with 

 diethyl sulfate and epichlorohydrin, inhibition of protein synthesis ac- 

 tually increases the frequency of mutations in bacteria (Strauss and 

 Okubo, 1960). 



Only few attempts have so far been made to correlate the chemical 

 specificity of alkylating agents with their mutagenicity. An example is 

 the trifunctional agent, triethylenemelamine, which reacts with free 

 pyrimidine bases; the reaction product is still mutagenic with thymine 

 but not with cytosine (Lorkiewicz and Szybalski, 1961). The authors 

 propose that triethylenemelamine is mutagenic by altering a DNA pre- 

 cursor into a mutagenic base analog. This is apparently not the only 

 effect since Fahmy and Fahmy (1958) reported that triethylenemelamine 

 induces dominant lethals in Drosophila by causing large chromosomal 

 alterations. 



Most extensively analyzed with respect to both chemical effects on 

 DNA and mutagenic effects are the methylating and ethylating agents. 

 Such agents are dimethyl sulfate (DMS), diethyl sulfate (DES), methyl 

 methanesulfonate (MMS), ethyl ethanesulfonate (EES), ethyl methane- 

 sulfonate (EMS). They all act as monofunctional agents even when 

 they cany two functional groups like DMS and DES since each group 

 alkylates separately. 



DNA can be altered in five different ways by these alkylating agents 

 (see Fig. 13). 



1. The strongest reaction is the alkylation of the phosphate groups of 

 nucleic acids, which has been measured for many alkylating agents 

 (Alexander, 1952; Reiner and Zamenhof, 1957; Stacey et al., 1958). The 

 phosphate triester thus formed is unstable and for the most part hydro- 

 lyzes to return the free alkyl group (Alexander and Stacey, 1958). If 



