HAAS, ET AL.: MUTATION INDUCTION IN BACTERIA 167 



the mutation frequency are said to be involved in mutation stabili- 

 zation. The frequency of mutation is therefore determined by the 

 effectiveness of the MS processes. The nature of this process is not 

 known; however, the fact that 5-HU will lower the induced muta- 

 tion frequency (presumably through incorporation into RNA) in 

 a process always correlated with MS suggests that stabilization 

 involves "templating" preparatory to macromolecular synthesis. 



If MS does not take place an antagonistic process occurs which 

 removes the potential mutation from pathways leading to muta- 

 tion induction. This process, which we have termed mutation fre- 

 quency decline, is enzymatically mediated (27, 5) and requires 

 energy, but not RNA or protein synthesis. Also DNA synthesis 

 is not required since the process occurs in the absence of thymine 

 with a thymine-requiring auxotrophic strain (5). Moreover, MFD 

 can be carried to completion without influencing either the timing 

 or the rate of subsequent syntheses definitely involved in mutation 

 induction. Therefore, the processes involved in either MFD or MS 

 must take place prior to the RNA and protein synthesis made 

 necessary by UV irradiation to further DNA replication (2, 5, 6, 

 10). These results would seem to eliminate hypotheses for MFD 

 based on modification of timing of DNA synthesis so as to give more 

 time for "decay of a premutational state" at a constant rate (13), or 

 disappearance of "mutagens" at a rate independent of the treat- 

 ment (14). 



Ultimately, the potential mutation is fixed in the cell and 

 no longer subject to conditions causing MFD. The striking correla- 

 tion between RNA synthesis and MF (measured by challenge pro- 

 cedures specifically blocking RNA or protein synthesis) leads us to 

 believe that MF is mediated by RNA synthesis. There appears to be a 

 relation between the amount of postirradiation RNA synthesized at 

 the time of CMP addition and the rate of DNA synthesis in the pres- 

 ence of the inhibitor, as well as the level of induced mutation 

 obtained. If the RNA and protein necessary for genetic replication 

 have not been formed prior to CMP addition, there is no DNA 

 produced and the potential mutation is lost through MFD. It is 

 probable that the RNA and protein involved in MF are identical 

 with that involved in postirradiation recovery of the DNA-synthesiz- 

 ing system. The experiments make quite clear that acquisition of 



