HAAS, ET AL.: MUTATION INDUCTION IN BACTERIA 171 



29. Zelle, M. R. 1955. Effects of radiation on bacteria. Chapter X in 

 "Radiation Biology", A. Hollaender, Ed. Book II, 365-430. New 

 York: McGraw-Hill Book Co, Inc. 



Comments 



Auerbach: Is the separation into mutation stabilization and mutation 

 frequency decline not an artifact, caused by your having used for base- 

 line mutation frequency on plates with an intermediate amount of 

 broth? If you had used mutation frequency on minimal medium as base 

 line, would not both phenomena have turned out to be the same — or 

 rather reverse sides of the same phenomenon? 



Haas: It may very well be that mutation stabilization and mutation 

 frequency decline are reverse sides of the same phenomenon, but they 

 most certainly are not the same thing; nor is the separation an artifact 

 caused by using mutation frequencies obtained on plates supplemented 

 with an intermediate amount of broth for the base line. We use the 

 intermediate level of broth supplementation so that we can demonstrate 

 and study both processes at the same time on identical samples, but 

 the phenomena of mutation stabilization and of mutation fixation, 

 as well as the relation between the two, can be demonstrated just as 

 well with plating on minimal medium. Similarly, the phenomena of 

 mutation stabilization and mutation frequency decline can be demon- 

 strated on broth-agar plates containing a much higher degree of broth 

 supplementation. 



It is quite probable that both mutation stabilization and mutation 

 frequency decline are affecting the same biosynthetic step in DNA 

 replication, but one process (stabilization) leads to a maximum fre- 

 quency of mutations (provided mutation fixation takes place before any 

 decline process occurs), while the other (mutation frequency decline) 

 leads to a less number of mutations. Therefore, the processes certainly 

 must be different. When one incubates irradiated cells in buffer con- 

 taining a high level of amino acids, no increase in the mutation fre- 

 quency will be obtained with incubation, and neither will there be any 

 mutation frequency decline. However, these incubated cells are still 

 fully susceptible to either phenomenon and if, after a period of say 

 30 minutes buffer-amino acid incubation, conditions suitable for meas- 

 uring mutation fixation (addition of minimal medium to the culture) 

 or conditions suitable for mutation frequency decline (addition of 

 chloramphenicol) are initiated, we find that both phenomena will take 



