HAAS, ET AL.: MUTATION INDUCTION IN BACTERIA 153 



um. When incubation in complete medium or M minus nitrogen 

 medium is allowed for short intervals before adding DNP intermedi- 

 ate levels of MS or MFD are obtained. Experiments in which high 

 levels of amino acids were added to the incubation medium subse- 

 quent to or during MFD indicate that the process is not reversible. 

 Once MFD has proceeded to any level, addition of amino acids will 

 not increase the mutation frequency obtained with subsequent 

 incubation beyond that level. 



Witkin (26, 27) has suggested that delay in DNA synthesis could 

 increase mutation since more time would be available for protein 

 synthetic processes involved in mutation induction prior to DNA 

 synthesis. On the other hand, Kimball, et al (13) propose that delay 

 in DNA synthesis would decrease mutation since more time would 

 be available for reversion of an unstable "pre-mutational" state to a 

 stable state not inducing a mutational change. 



It has previously been demonstrated that UV will cause a delay 

 in DNA synthesis (12), and several investigators (2, 6, 10) have since 

 shown that resumption of DNA synthesis following UV requires 

 prior synthesis of RNA and protein. In our experiments several inhib- 

 itors which block RNA or protein synthesis prior to initiation of 

 DNA synthesis in a UV-irradiated culture were found not only to 

 inhibit DNA synthesis, but also to promote a decided decrease in the 

 induced mutation frequency (5). If a quantitative correlation could 

 be established between the effects of these agents on mutation fre- 

 quency and their effect in delaying DNA synthesis, then the hypothe- 

 sis advanced by Kimball, et al (13), would be supported. But if MFD 

 occurs independently of the effect on DNA synthesis, then the 

 hypothesis that MFD is an active enzymatically controlled process (3) 

 would be more likely. Several experiments designed to give evidence 

 as to the most likely of these alternatives were performed. These 

 experiments measured the effect of increasing periods of postirradi- 

 ation treatments blocking RNA or protein synthesis on the subse- 

 quent DNA synthesis and mutation frequency. Absence of nitrogen 

 source, absence of tryptophan for the tryptophan-requiring auxo- 

 troph, presence of 6 aza uracil, and presence of CMP were used as 

 inhibitors. With all experiments the results demonstrated that condi- 

 tions sufficient in duration to lead to lower mutation frequency 

 through blockage of RNA or protein synthesis produce no significant 



