33. NUCLEIC ACIDS OF THE BACTERIAL VIRUSES 193 



c. Inactivation by Decay of Phosphorus-32 



It was demonstrated by Hershey, Kamen, Kennedy, andGest, 61 thatT2 

 phage, heavily labeled with P 32 , is inactivated upon storage at a rate which 

 makes the effect clearly the result of P 32 disintegration within the phage 

 DXA. An equivalent jS-irradiation from an external source would not pro- 

 duce more than 3% of the observed inactivation, so the effect must be a 

 consequence of the transmutation taking place in the DXA chains. 



The kinetic and quantitative aspects of this experiment have made it 

 clear that one P 32 disintegration could inactivate a phage, but that not 

 every one does, and that at 4° any P 32 disintegration has one chance in ten 

 of inactivating the virus particle. 51 • 52 This efficiency factor of Ko has been 

 shown to be valid for a number of phages, excepting those of the minute 

 group. 



This phenomenon — known as "P 32 suicide" — can be used to determine the im- 

 portance of the integrity of the parental, progeny, or host DNA molecules at various 

 stages during infection. By halting the infective process at a given time, freezing the 

 infected cells for various periods of time to allow P 32 decay, then thawing the cells 

 and determining by assay the effect of the P 32 decay upon the fate of the infection, 

 one can obtain suggestive evidence concerning the importance at the given time of 

 various P 32 -containing components for the success of the infection. 



Conversely if the specific activity of the P 32 is known and an efficiency factor of 

 1 lo is assumed to hold, one can make a good estimate from the "suicide" rate of the 

 DNA content of a particular virus. Other ingenious uses of this phenomenon will be 

 developed later. 



II. Nucleic Acids of the T-Even Phages 



1. Structural Aspects 



a. Macromolecular Properties; Bipartite Nature 



The DNA content of a T-even phage particle is of the order of 120 X 

 10 6 Daltons (Table I). The macromolecular character of this DXA is a 

 matter of controversy, even of paradox, at the present time. Several lines 

 of evidence suggest that this DX'A should be considered to be bipartite, 

 consisting of one very large molecule comprising 36% of the DX'A (molec- 

 ular weight approximately 45 X 10 6 ) and several smaller molecules of DXA 

 of weight 12 X 10 6 and less. Conflicting evidence is available however which 

 suggests that this DX'A consists of relatively homogeneous molecules of 

 molecular weight 14 X 10 6 . 



(1) Autoradiographic Evidence. The most direct evidence of the bipartite 



51 A. D. Hershey, M. D. Kamen, J. W. Kennedy, and H. (lest, J. Gen. Physiol. 34, 

 305 (1951). 



52 G. S. Stent and C. R. Fuerst, J. Gen. Physiol. 38, 441 (1955). 



