168 MUTATION AND PLANT BREEDING 



capacity to synthesize DNA rather than DNA synthesis per se is 

 necessary lor mutation fixation, since DNA synthesis during CM Is- 

 chial lenge is not necessary. 



A number of hypotheses have been advanced in regard to the 

 mechanism of mutation induction by UV. However, it is quite 

 difficult to explain most of the postirradiation data on the basis of 

 "direct-induction" theories. In our opinion, the hypothesis for the 

 unstable portion of UV-induced mutation which best accommodates 

 the experimental data is that which we have previously advanced 

 (4). According to this hypothesis, DNA synthesis involves transfer of 

 information from the parental DNA to the daughter DNA through 

 an RNA-protein intermediate. Incorporation of a UV-modified pre- 

 cursor into the RNA leads to a "copy error" involving substitution of 

 a different nucleotide pair or perhaps deletion of the usual nucleotide 

 pair in the subsequently formed daughter DNA. While little experi- 

 mental support at the molecular level exists for this indirect mecha- 

 nism of DNA replication, the same situation also exists for theories 

 of a more direct mechanism. There are also several indications that, 

 at least during phage DNA replication, the genetic information is at 

 certain stages carried in some structure or molecule other than the 

 DNA (16, 24). 



Our findings thus far for X-ray-induced mutations can be 

 briefly summarized as follows: Approximately half of the X-ray- 

 induced reversions of certain amino acid-requiring auxotrophs of 

 E. Coli are sensitive to CMP, or to 6-AU. A short period of incu- 

 bation involving protein synthesis is required for this sensitivity to 

 become manifested. These mutations require RNA, DNA, and 

 protein syntheses for induction and are apparently induced in a 

 manner comparable to that for the major portion of UV-induced 

 reversions. It is probable that these mutations are induced dur- 

 ing replication of the daughter DNA. The other fraction of X-ray- 

 induced mutations are not lost by MFD processes when the cells 

 are incubated in the presence of CMP or 6-AU. These mutations 

 are expressed when the culture is plated on M agar after a short 

 period of incubation in complete medium, and during this incu- 

 bation no detectable DNA synthesis occurs. This suggests that the 

 second fraction of mutations is induced by X-ray in the gene, and 

 that DNA replication is not necessary for induction. However, RNA 



