33. NUCLEIC ACIDS OF THE BACTERIAL VIRUSES l'.tl 



after infection. This lytic process can be largely inhibited by appropriate 

 means (lysis inhibition) 36 in which case the accumulation of mature phage 

 particles continues for several hours. 



b. Phenomena Observed with Ultraviolet Irradiated T-Even Phages 



Many of the experiments to be described make use in varied ways of 

 bacteriophage that have been irradiated with ultraviolet light. Usually 

 the radiation employed is the unfiltered output of a germicidal lamp and 

 is presumed to be, in large part, energy of wavelength of the mercury res- 

 onance line at 2537 A. This radiation produces lesions of unknown nature 

 in the bacteriophage. A variety of evidence, all unfortunately indirect, 

 indicates that these lesions are very largely localized in the DXA of the 

 phage particle. 



These lesions are not homogeneous. There are at least two classes of 

 lesions which may be differentiated in a physicochemical sense as photo- 

 reactivable and nonphotoreactivable. 37 " 39 



Lesions may also independently be distinguished as to whether they 

 affect DNA function (functional change) or DXA replication (genetic 

 change) or both. It is conceivable that a lesion may prevent the proper 

 function of a segment of DNA but would permit, at the time of replication, 

 formation of a new undamaged, functional DXA. (Symonds and McCloy 

 have presented evidence that the photoreactivable lesions in phage T2 are 

 of the exclusively functional type. 40 ) 



Following this concept, lesions may also independently be distinguished 

 as to whether they affect a "critical" or a "noncritical" function. 41 " 43 A "criti- 

 cal" function in this sense is one that must be expressed before replication 

 of phage DX'A is possible. Exclusively, functional damage to a "noncritical" 

 locus could then be, in effect, repaired at the first replication, while similar 

 damage to a "critical" locus would, in single infection, prevent replication. 



The genetic damages might or might not prevent function but would 

 invariably, in single infection, result in either a total block to replication 

 or in an error at replication. The error might result in a mutation or might 

 be lethal. 



These lesions are recognized in the first instance by the fact that they 



36 A. H. Doermann, J . Bacteriol. 55, 257 (1948). 



37 R. Dulbecco, J. Bacteriol. 59, 329 (1950). 



38 R. Dulbecco, in "Radiation Biology'' (A. Hollaender, ed.), Vol. II, p. 455. Mc- 

 Graw-Hill, New York, 1955. 



39 J. Jagger, Bacteriol. Revs. 22, 99 (1958). 



40 N. Symonds and E. W. McCloy, Virology 6, 649 (1958). 



41 N. A. Barricelli, Acta Biotheoretica 11, 107 (1956). 



42 R. H. Epstein, Virology 6, 382 (1958). 

 « D. R. Krieg, Virology 8, 80 (1959). 



