PHOTOREACTIVATION 457 



tivated, only the S — S groups contiguous to reduced DPN molecules 

 being reduced; and reactivation is a reaction of approximately first order. 



Reactivation of triosephosphate dehydrogenase by light may well be 

 a model for photoreactivation. The latter phenomenon differs appar- 

 ently because it affects only functional ultraviolet changes related to 

 growth, the simplest of which is probably the formation of adaptive 

 enzymes (Swenson and Giese, 1950) ; however, this may be so because 

 photoreactivation is an indirect process, involving substances commonly 

 found in the cytoplasm of cells, and because the most outstanding dam- 

 ages produced by ultraviolet are those affecting growth. 



In this review the observations made on different organisms will be dis- 

 cussed separately. By far the most detailed cjuantitative measurements 

 have been made with the coli phages of the T series. These will be pre- 

 sented first, and some theoretical notions verifying these data will be 

 developed. ]\Ieasurements of photoreactivation in bacteria are almost as 

 detailed as those on phage, and these data, too, can be accounted for on 

 the basis of a few simple assumptions. However, it is remarkable that 

 the formal schemes developed for photoreactivation in phage and bacteria 

 are quite different and seemingly incompatible. Since it does not seem 

 reasonable to assume radically different mechanisms for so obviously 

 similar phenomena, both theories must be viewed with suspicion, and 

 a deeper interpretation q{ all the data must be sought. In the remainder 

 of this review the scattered data which have been reported for other 

 organisms will be briefly summarized. 



3. PHOTOREACTIVATION IN BACTERIOPHAGES 



Photoreactivation has been studied in the bacteriophages of the T 

 group active on Escherichia coli strain B (Dulbecco, 1949, 1950). 



3-1. CONDITIONS OF INACTIVATION 



The bacteriophages used are very stable, and can be kept for months 

 as suspensions in buffered saline solutions. Such suspensions are inac- 

 tivated by exposure to an ultraviolet source, which in the majority of the 

 experiments has been a G.E. germicidal lamp, emitting 80 per cent of its 

 energy in the wave length 2537 A. 



After irradiation a certain fraction of the particles is inactive. The 

 logarithm of the fraction that is still active after a given ultraviolet dose, 

 plotted versus the dose, gives the so-called "survival curve," whose shape 

 is characteristic for a given phage. 



3-2. xMETHODS FOR STUDYING PHOTOREACTIVATION 



If equal samples from a suspension of phage irradiated with ultraviolet 

 are plated with sensitive bacteria on two equal plates and one is incubated 



