Bacteria : Clones and Mutation 



337 



Significant differences in mutation rate were 

 observed when different components of the 

 nutrient medium were made the Hmiting fac- 

 tor for growth. The highest rate was obtained 

 when tryptophan was the growth-controlling 

 factor and the lowest rate was obtained when 

 lactate was the growth-controlling factor. 



The fact that limiting different nutrients has 

 a significant effect upon spontaneous muta- 

 tion rate is a clear indication that this rate 

 depends upon the physiological or biochemi- 

 cal state of the organism. This view is sup- 

 ported by the effect of temperature changes 

 upon the spontaneous mutation rate (see 

 p. 200). This also suggests that if chemical 

 substances are added to a culture of E. coli 

 growing in a medium with one essential nutri- 

 ent limited in supply, a pronounced mutagenic 

 action might be obtained. A number of sub- 

 stances were chosen to be tested (some of 

 which were already known to be mutagenic 

 in other organisms) and were used in con- 

 centrations that produced no appreciable 

 killing of bacteria. 



One group of substances tested for capacity 

 to induce mutations to T5 resistance, when 

 nutrition was tryptophan-limited, was pu- 

 rines and purine derivatives. Many were 

 mutagenic. The most mutagenic was caffeine; 

 theophylline was nearly as effective; aza- 

 guanine was mutagenic, and so also, to a 

 lesser degree, was adenine itself. In contrast, 

 no pyrimidines or their derivatives were muta- 

 genic under the same conditions. 



It was also found ^ in this system that if 

 purine ribosides, like adenosine or guanosine, 

 were added to the medium containing any one 

 of several purine mutagens, that there was 

 complete suppression of the mutagenic ac- 

 tivity. Thus, for example, adenosine com- 

 pletely suppresses the mutagenicity of adenine 

 or caffeine. The purine nucleosides were 

 clearly acting as antimutagens, just as anoxia 

 or catalase are antimutagens so far as chro- 

 mosomal breakage (p. 179) or point muta- 

 ^ A. Novick (1956); see reference at end of Chapter. 



tions (p. 201) produced by X rays are con- 

 cerned, and were not acting as selective 

 agents against induced mutants. On the 

 other hand, pyrimidine ribosides, deoxy- 

 adenosine, and deoxyguanosine were either 

 not at all antimutagenic to purines and their 

 derivatives, or much less efficient than the 

 purine ribosides. None of the purine ribo- 

 sides had any antimutagenic effect on ultra- 

 violet or gamma radiation treatments. 



Theophylline is mutagenic, as mentioned, 

 under aerobic conditions. But under anaero- 

 bic conditions theophylline is not mutagenic. 

 One need not be satisfied just to call anaero- 

 biosis antimutagenic, for it has been found 

 that adenosine is present in significant concen- 

 trations in bacteria growing anaerobically but 

 is not present in detectable quantities in bac- 

 teria growing aerobically. Apparently, un- 

 der anaerobic conditions, adenosine is a nor- 

 mally present antimutagen that acts against 

 the effect of purines added in the medium. 

 It should be noted that the ultraviolet induced 

 mutation rate is unaffected by anaerobiosis, 

 and gamma radiation is less effective only 

 because of the reduction in oxygen; this is 

 consistent with the finding, already men- 

 tioned, that added adenosine has no anti- 

 mutagenic effect on either mutagen. 



One may ask whether the spontaneous mu- 

 tation rate itself is reduced by the presence 

 of purine ribosides in the medium. It was 

 found that adenosine and guanosine, but not 

 the pyrimidine ribosides uridine and cytidine, 

 suppress the spontaneous mutation rate. 

 With adenosine in the medium, the spontane- 

 ous rate could be reduced to about one third 

 its original value. Moreover, the spontane- 

 ous rate is lower under anaerobic than aerobic 

 conditions, as would be expected from the 

 metabolic production of adenosine. Finally, 

 it may be noted that all the purine mutagens 

 increase the mutation rate to T5 resistance 

 more than to T6 resistance, while the reverse 

 is true for ultraviolet and gamma radiation. 



From these results it seems reasonable to 



