274 



H. H. PLOUGH 



An actual test of artificially made mixtures of the parent strain and one 

 cysteine requiring mutant as screened by the media is shown in Table 17.3. 

 The data show that a mixture of 90 per cent wild and 10 per cent mutant 

 still gives a greater number of wild survivors after penicillin screening than 

 does a mixture having 10 per cent wild and 90 per cent mutant. For the 

 actual experiments reported in Table 17.1 the proportion of mutants to un- 

 mutated wild type even after 24 hours of growth in complete broth is one 

 in many thousands, rather than 10 per cent to 90 per cent. So it seems justi- 

 fied to consider the percentage of mutants and wild type as an index of muta- 



TABLE 17.3 



EFFECT OF GROWTH IN COMPLETE MEDIUM FOLLOWED 

 BY PENICILLIN SCREENING ON ARTIFICIAL MIX- 

 TURES OF CONTROL (533) AND A CYSTEINE AUXO- 

 TROPH (533-169) 



tion frequency in comparing X-ray dosages. The trend in Figure 17.3 sug- 

 gests a sigmoid curve rather than a straight line as Hollaender (1948) has 

 shown for ultraviolet induced visible mutations in fungi. Essentially the 

 same interpretation can be drawn from a comparison of the number of differ- 

 ent mutations found at the successive X-ray dosages. Much more extensive 

 data are now available showing the relation between mutation frequency 

 and both X-radiation and ultraviolet dosages and they will appear in another 

 publication. In general they all bear out the conclusion that the frequency 

 of auxotrophic mutations is directly correlated with radiation dosage as is 

 true for gene mutation in other organisms. 



A rather interesting result of comparison of these percentages of mutants 

 present after penicillin screening is that the most frequent class changes from 

 the lower to the higher dosages. Thus after 11,000 roentgens, a cysteine auxo- 

 troph is the most frequent, while after 57,000 r it is a histidine requirer. 

 Perhaps we are dealing with a specific effect of dosage or conceivably with 

 a differential effect of wave length, but until the complex nature of the cys- 

 teine mutants are more fully understood it is unwise to attempt too definite 

 an interpretation. 



RECOMBINATION TESTS IN SALMONELLA 



Much interest has been excited among geneticists as well as bacteriologists 

 by Lederberg's proof that mixtures of multiple mutant stocks of the K12 



