158 MUTATION AND PLANT BREEDING 



Schwartz and Strauss (21) have shown that incubation of UV- 

 irradiated E. coli strain WP2 in the presence of tryptazan (a trypto- 

 phan analogue which is incorporated into protein) will result in a 

 decline in mutation frequency. They suggest that this decline is 

 caused by protein synthesis utilizing tryptazan. Their results consti- 

 tute significant biochemical evidence implicating protein synthesis 

 in mutation induction. If protein formed in the presence of trypta- 

 zan and subsequent to mutation fixation were intimately involved 

 with RNA in the replication of genetic DNA, then "nonfunctional" 

 protein would probably prevent utilization of the corresponding 

 RNA in DNA replication and in so doing use up the mutagenic 

 precursors. 



All these experiments indicate that MF is closely correlated with 

 the RNA synthesis, and that the mutations are established in some 

 structure or form before any measurable DNA synthesis takes place. 

 They support our previously presented hypothesis (4) that RNA and 

 protein syntheses are intimately involved in replication of genetic 

 DNA; and that the RNA, modified by incorporation of a radiation- 

 altered precursor, in some manner leads to a corresponding modifi- 

 cation in newly formed DNA. This implies that induced mutations 

 first appear in the daughter DNA initially synthesized after irradi- 

 ation, and that a period of time is thus necessary between UV treat- 

 ment and establishment of the mutation in the genome. During this 

 interval manipulations interfering with RNA and protein syntheses 

 can prevent the potential mutation from being induced. 



It is of considerable importance to determine at what point in 

 the sequence of postirradiation events the induced mutation is first 

 capable of being phenotypically expressed. Establishment of this 

 point would differentiate processes involved in gene synthesis from 

 those of enzyme synthesis involved in phenotypic expression. Dur- 

 ing the course of experiments with reversion of the tryptophan 

 requirement of E. coli strain WP2, it became apparent that, dur- 

 ing the early stages of postirradiation incubation, if the bacteria 

 were plated on M agar rather than on broth-supplemented M agar, 

 much lower induced mutation frequencies were obtained. After 90 

 minutes postirradiation incubation in complete medium, however, 

 the mutation frequency obtained was the same on both M agar and 

 broth-supplemented M agar. It seemed probable that the lower level 



