ON THE MECHANISM OF LETHAL ACTION OF 

 X-RAYS IN ESCHERICHIA COLI K12 



H. MARCOVTCH 



[with flic technical assistcuice of Mile B. Vai/ne) 



Service de Kadiobiologie et de Cancerologie de Vlnstitut Pasteur, Paris, 



France 



SUMMARY 



Crosses have been utilized in order to elucidate the mechanism of the lethal 

 effect of X-rays on Escherichia coli K12. These bacteria possess two to three 

 nuclear bodies and are killed according to a one-hit dose-effect relationship. The 

 experiments were designed to search for the existence of X-ray dominant lethal 

 mutations in one of the nuclei, transferable to the recombinants. No svich muta- 

 tion was detected. 



Among the lesions which are responsible for permanent inhibition of 

 division in irradiated cells, those of the nucleus are the best known and 

 the most extensively studied. Their role seems to be predominant. In 

 higher organisms, tissue damage is essentially the consequence of 

 chromosomal disorders, not compatible with the normal sequence of 

 mitotic events. The high radiosensitivity of the nucleus is clearly de- 

 monstrated in irradiation experiments of Habrobracon eggs with a-rays 

 (Rogers and von Borstel, 1957; von Borstel and Rogers, 1958). Mam- 

 malian somatic cells are killed by X-rays according to a sigmoidal dose- 

 effect relationship; two events are necessary to destroy the colony 

 forming ability (Puck and Marcus, 1956; Puck et al., 1957). 



In haploid yeast, the X-rays survival curves are exponential, but in 

 diiDloids they are two-hit sigmoidal ones (Latarjet and Ephrussi, 1949). 

 It has been possible to have direct evidence of the existence of two 

 kinds of genetic lesion. The more frequent are recessive lethal mutations 

 responsible for the sigmoidal shape of the diploid survival curves. The 

 others are dominant lethal mutations (Mortimer, 1955). 



In these examples, the lethal effect of X-rays may chiefly be attrilni- 

 ted to lesions of the nuclear material. 



In Escherichia coli, the survival curves are, in practically all in- 

 stances, exponential. This observation does not depend on the stage 

 of the cultures or on the composition of the growth medium. The 

 bacterial cells possess a variable number of nuclear bodies, according 



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