HAAS, ET AL.: MUTATION INDUCTION IN BACTERIA 151 



involved in decline are enzymatic was obtained by studying the 

 decline rate at different incubation temperatures ranging from 6° to 

 37° C. This established that the mutation frequency decline (MFD) 

 process has a Q K1 of approximately 2. The hypothesis was suggested 

 that when amino acids or a nitrogen source were absent, some enzy- 

 matic process removes the potential mutation before it is "fixed" in 

 the genetic structure. Amino acids might then serve in "stabilization" 

 of the pre-mutation or mutagen, removing it from susceptibility to 

 MFD until those processes leading to incorporation in the genetic 

 structure could be completed. 



The role of CMP in interfering with mutation induction was 

 investigated and found to promote MFD in a manner completely 

 analogous with that caused by nitrogen or amino acid deprivation 

 (4). It was thus considered that CMP acts to prevent some process of 

 "mutation stabilization" (MS) involving amino acids, thus allowing 

 MFD to proceed. Figure 1 compares the time course and relationships 

 of MFD and MS in regard to tryptophan-requirement reversions in 

 E. coli strain WP2. When the cells are plated immediately after irradi- 

 ation onto broth-supplemented M agar the mutation frequency 

 attained at the UV dose used was about 40 prototrophs per 10 6 sur- 

 vivors. However, this level of nutrient broth supplementation affords 

 amino acid concentrations much lower than optimum for maximum 

 mutation response as is apparent from the Min + AA curve in Figure 

 1. Here the cells were incubated following irradiation in M medium 

 supplemented with higher levels of amino acids for increasing time 

 intervals before plating onto the limiting broth-supplemented M 

 agar. It is apparent that when the cells are incubated for 30 to 40 

 minutes with excess amino acids, the induced mutation frequency 

 obtained on subsequent plating almost doubles. Therefore, the amino 

 acids have been taken into the cell and are involved in processes 

 leadino to mutation induction. On the other hand, when the cells 

 are incubated in medium containing no amino acids or other nitrogen 

 source (Min — N curve in Figure 1), or if CMP is added to amino 

 acid-supplemented medium (Min + A A + CHL curve in Figure 1), 

 MFD leads to a low mutation frequency within 20 minutes. Oxidative 

 phosphorylation is probably required for MS since, if the irradiated 

 cells are incubated in complete medium to which dinitrophenol 

 (DNP) has been added, MS does not take place (Min + AA + DNP 



