174 MUTATION AND PLANT BREEDING 



tion of an isolated transforming principle (6) in contrast to the 

 action of nitrous acid, I believe ultraviolet-induced mutation must 

 be a more complex process.) Doctor Haas, and others (17), have 

 demonstrated that synthetic processes intervene between the pri- 

 mary ultraviolet absorption product and the final mutant state. 



The process of ultraviolet-induced mutation is in fact a very 

 special case. A portion of the ultraviolet-induced changes can be 

 reversed by visible light and this photoreactivation is unique to 

 ultraviolet-induced mutation. (The photoreactivation process is 

 itself only partially understood. It is known to be enzymatically 

 mediated.) While it is established that protein synthesis, or some 

 closely related process, is required for the fixation of ultraviolet- 

 induced mutations, it is equally evident, both from the work report- 

 ed by Doctor Haas at this symposium and from other work, that 

 protein synthesis is not a general requirement for the fixation of 

 mutations. Bromouracil is mutagenic for bacteriophage even when 

 introduced along with chloramphenicol as an inhibitor of protein 

 synthesis (7). Protein synthetic processes do not play an important 

 role in the fixation of those mutations induced by the alkylating 

 agent ethyl sulfate, although they may be required for the expres- 

 sion of the mutations (14). The frequency of ethyl sulfate-induced 

 mutation is not reduced when E. coli is treated in the presence of 

 any one of the inhibitors methyltryptophan, ethionine, or 6-mer- 

 captopurine nor does post-treatment incubation in buffer, or in buffer 

 containing these inhibitors, seem to alter the mutation frequency 

 significantly. 



According to the ideas presented by Doctor Atwood earlier 

 in this symposium, the genetic material is a linear sequence of 

 deoxyribonucleotides. The sequence forms a code which is deci- 

 phered by the cell and the process of deciphering represents gene 

 action. Base analogues incorporated into the DNA lead to muta- 

 tion only by increasing the probability that a mistake will be made 

 on duplication leading to the substitution of one nucleotide pair 

 for another. DNA containing a base analogue is not of itself mutant — 

 bacteriophage with bromouracil completely substituted for thymine 

 can produce normal progeny (8). The mutation process can only be 

 considered complete when a changed DNA (itself able to replicate) 

 containing only normal bases is formed as a result of a replication. 



