Section 5 — Mutagenesis 



results suggest that X-ray mutagenesis, in the 

 example studied here, is exerted directly on the 

 chromosome, and that u.v. mutagenesis involves 

 a rather long-lasting cytoplasmic step before the 

 mutant character is integrated in the chromos- 

 some. 



5.23. The Effect of U.V.-Light and X-rays on the Varia- 

 bility of Bacillus licheniformis. F. Paleckova 

 (Prague, Czechoslovakia). 



The relationship between irradiation dose, 

 killing effect and production variability of a 

 bacitracin-producing strain of B. licheniformis 

 was studied. The organism was exposed to u.v.- 

 light for 4, 8, 16, 32 sec; the survival curve was 

 sigmoidal. In experiments with X-ray treatment 

 doses of 200,000 r, 400,000 r, 600,000 r were used. 

 The survival curve was linear. The effect of these 

 mutagenic factors on the variability of the anti- 

 biotic-producing ability of the strain was studied 

 and submitted to statistical analysis. 



5.24. A Region of the Chromosome of Escherichia coli 

 K12, abnormally Resistant to Ultraviolet. 



F. Joset and H. Marcovich (Paris, France). 



Studies dealing with the effects of radiations 

 on genetic recombination in E. coli K 12 have 

 shown that a region of the male chromosome is 

 abnormally resistant to ultraviolet and a radia- 

 tions. This region, located between the determi- 

 nants for Tryptophane and Histidine, represents 

 about one-fourth of the whole chromosome. 



No genetic markers have yet been found in 

 this region, as compared to about 100 for the 

 rest of the chromosome; assuming a random 

 distribution of the markers this fact might have 

 a biological signifiance. 



Two suggestions can be made which could 

 explain the resistance to u.v. and the absence of 

 genetic markers: 



1. This region might be almost completely 

 empty of DNA. This hypothesis seems to be 

 ruled out by 32 P labeling experiments, which 

 show a normal sensitivity to radio-phosphorus 

 per unit length. These experiments have shown 

 the existence of a second series of lesions in- 

 duced by 32 P. These lesions can be transferred to 

 the female during a cross, and prevent the in- 

 tegration of the labelled material in the zygote 

 chromosome. 



2. If not absent, the DNA of this region might 

 have an abnormal base composition. The mole- 



cular basis of u.v. damage is thought to be mostly 

 a dimerization of thymine. As this region is 

 relatively insensitive to u.v., one may think that 

 thymine dimers are less likely to be formed, 

 either because of a low thymine content, or be- 

 cause of the absence of neighboring thymine- 

 molecules. 



Experiments using base analogues such as 5- 

 bromouracil, are being done to test this possi- 

 bility. 



5.25. The Effect of Temperature on the Response to 

 Ultraviolet of Neurospora crassa. B. J. Kilbey 

 (Currie, Great Britain). 



The temperature of the conidial suspension 

 during ultraviolet irradiation has been found to 

 influence the mutagenic response of the strain 

 K 3/17 ad-3A 38701, inos 37401.W A dose of 

 ultraviolet administered at 3-4°C may produce 

 two to three times the number of adenine rever- 

 sions compared with a similar dose administered 

 at temperatures between 25°C and 30'C. Survival 

 and inositol reversions do not appear to be 

 greatly influenced by this difference in tempera- 

 ture. 



The evidence at present available suggests that 

 a temperature sensitive recovery process is acting 

 to repair adenine reverse mutations. Interest is 

 centred on the specific nature of this process and 

 it is hoped that this may be clarified by the study 

 of other ad-3A mutants and inos mutants which 

 respond to ultraviolet light, and by an examina- 

 tion of the effect of varying the experimental 

 conditions. 



1 . A full description of the characteristics of this 

 strain may be found in Zeit. fiir Vererbungs- 

 lehre 93, 356-365, 1962. 



5.26. Genetic Studies of a Mutation to Radioresistance 

 induced by 32 P decay in Saccharomyces cere- 

 visiae. E. Moustacchi, H. Hottinguer-de 

 Margerie, and H. Marcovich (Paris, France). 



Different agents ( 32 P decay, ultraviolet light, 

 mustard-gas, ionizing radiations) induce in hap- 

 loid Saccharomyces cerevisiae a mutation to- 

 wards resistance to the lethal action of ionizing 

 radiations. DNA content per cell of these resistant 

 strains show that they are still haploid. This is 

 confirmed by the fact after that sporulation of 

 diploid strains from a cross between a radio- 

 sensitive and a radioresistant strain the viability 



63 



