OLE MAAL0E AND KARL G. LARK 



prophage state seems to involve an interaction between phage material and a bacter- 

 ial nucleus (Murray, 1953; Lederberg, 1953; Jacob and Wollman, 1953; Appleyard, 

 1953). If only one phage particle enters the cell the probability of such an interaction 

 occurring before unrestricted phage multiplication begins is rather low; it has been 

 shown, however, that this probability, i.e. the lysogenization frequency, can be in- 

 creased from 15 to 20 per cent, up to about 80 per cent, by increasing the number of 

 phage particles per bacterium (Boyd, 1950; Lwoff, Kaplan and Ritz, 1954). Our 

 experiments show that an increase in lysogenization frequency can be caused also 

 by increasing the number of nuclei per cell. From these observations it appears 

 probable that the lysogenization frequency is determined by the number of en- 

 counters between the entering phage particle, or particles, and the nuclei of the cell, 

 which occur within a limited time after infecting the cell. In our experiments the 

 conditions of infection were such that 80 to 90 per cent, of the infected bacteria re- 

 ceived one phage particle only; and in such experiments nuclear doubling, through- 

 out a culture, seems to cause a doubling of the lysogenization frequency. 



What remains to be interpreted is the gradual decrease in lysogenization frequency 

 which is observed in the synchronized cultures during the periods while little or no cell 

 division takes place (see Figure 2). We believe that just after the 37 C. period when 

 the lysogenization frequency has reached its maximum, all cells have doubled their 

 nuclei; during the following 20 to 25 minutes the cells prepare for division, presum- 

 ably by developing some kind of internal separation. Gradually all the cells would 

 thus in a functional sense become double cells. In this way we would pass from a 

 situation in which nearly all cells have, say, four nuclei within one functional unit, 

 to a situation in which nearly all cells consist of two functional units, each with two 

 nuclei. In our lysogenization experiments each infected cell unit, including the 

 hypothetical double cells, adsorbed one phage particle only, and in the case of a 

 double cell only one compartment of the cell would therefore be infected. It follows, 

 from what was said above, that the postulated segregation process by which the 

 number of nuclei per functional unit is reduced to one-half should be accompanied 

 by a similar reduction in lysogenization frequency. This analysis of the lysogenization 

 curve of Figure 2 brings us back to the idea that nuclear division is not immediately 

 followed by cell division, which seems to occur only after a long period of preparation. 



Acknowledgements. We wish to express our thanks to Dr. Lwoff for making available 

 to us the bacterial strains and the phage strains used, and for pointing out some of 

 their great advantages. We also wish to thank Dr. Kauffmann for generous gifts of 

 specific antisera. We are indebted to Dr. Hotchkiss and Dr. Zeuthen for detailed and 

 valuable information about their work prior to its publication. We are grateful to 

 Mr. O.' Rostock for expert assistance in the execution of most of the experiments. 



REFERENCES 



Appleyard, R. K. (1953). Cold. Spr. Harb. Symp. Qiiant. Biol. 18, 95. 



Bentzon, M. W., Maaloe, O. and Rasch, G. (1952). An analysis of the mode of 



increase in number of intracellular phage particles at different temperatures. 



Acta path, microbiol. scand. 30, 243. 



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