VIRAL FUNCTIONS: ORDER AND DISORDER 



Inducing agents. Ultraviolet light has an inducing effect but is 

 not the only inducer. A number of physical or chemical agents are 

 also inducers — among them, X rays, gamma rays, nitrogen mustard, 

 organic peroxides, epoxides, and ethylenimines. All these agents 

 are known to be mutagenic and oncogenic, that is, cancer-inducing. 

 The fact that inducing agents are mutagenic is not a great help: 

 induction of a lysogenic bacterium is not homologous to a mutation. 

 But the fact that inducing agents are able to initiate malignant growth 

 is rather exciting. Perhaps induction will help us to understand 

 cancer, but the reverse is certainlv not true. Anyhow, the theory 

 is that the inducing agents depress or block the synthesis of the 

 repressor. As a consequence, the level of the repressor decreases 

 below a given threshold. Since the prophage is no longer repressed, 

 the vegetative phase is initiated. 



A noninducible prophage could be a prophage which manufac- 

 tures more repressor, or a more stable repressor, or ^\'hich has a 

 greater affinity for the repressor. This problem will be discussed later. 



Zygotic induction. The prophage A of Escherichia coli K12 is 

 located on the bacterial chromosome close to one of the genes con- 

 trolling the utilization of galactose, which enters into the female at 

 about the 25th minute after conjugation. 



If both male and female are lysogenic, nothing happens after 

 conjugation, a fact that is worth while noticing. If only the female 

 is lysogenic, nothing happens either. But if a lysogenic male mates 

 with a nonlysogenic female, things are quite different (Figure 30). 

 As soon as the prophage attached to the chromosome of the lyso- 

 genic male has penetrated into the nonlysogenic female, the vegeta- 

 tive phase is initiated, bacteriophage particles are produced, and the 

 female is lysed. Thus, phage development is induced when, as a 

 result of conjugation or zygosis, an inducible prophage penetrates 

 into a nonlysogenic cytoplasm. This is zygotic indnctiov. 



These data are consistent with the hypothesis that in the lysogenic 

 male bacterium the expression of the viral functions is blocked by 

 a cytoplasmic repressor. When the prophage of the lysogenic male 

 enters the nonlysogenic female, repression ceases because the cyto- 

 plasm of the female is devoid of repressor, and viral functions are 

 expressed. How could the repressor act? The antibiotic chlor- 

 amphenicol is known to block the synthesis of proteins. If bacterial 



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