45() HADIXTION BIOLOGY 



is the ro\orsal of the inaclivation, the incaiiiii^ of iiiactivation iiiuhI he 

 clarilirtl. Tlic clcliiiition is a rolafivc one: a microorganism after treat- 

 ment witli a radiation is inaetixc if in ^iven standard conditions it is not 

 al)le to give rise to inde(initemviltii)lication. 'I'his is tested by determining 

 whether or not indefinite growth can be obtained by placing the organism 

 immediately after irradiation in darkness and at optimum temperature 

 into a nutrient medium. Growth is considered absent if no visil)le colony 

 is formed. A fraction of the organisms judged inactive by this technicjue 

 may be judged active if the technitiue is changed; examples are known in 

 bacteria in which the type of growth medium used or the temperature of 

 incubation afTects the fraction of organisms able to give rise to detectable 

 growth. In bacteriophages the criterion is still more imperfect since an 

 active phage particle can develop a colony only if the first sensitive bac- 

 terium infected by it bursts within a given time of incubation. In spite 

 of this imperfection, a fairly precise method for the study of reactivation 

 phenomena can be developed by strict standardization of techni(iue. 



2. HISTORICAL NOTE 



Photoreactivation was discovered and its generality appreciated by 

 Kelner (1949a), who observed reversal by visible light of ultraviolet inac- 

 tivation in spores of Streptomyces griseua. Previous observations of the 

 phenomenon can be found in the older literature, the most pertinent of 

 which is an observation by Whitaker (1941-42) on the ability of visible 

 light to counteract the action of ultraviolet on Fucus eggs. A review of 

 older observations has been given by Kelner (1950b). 



At this time the chemical and biological mechanisms involved in ultra- 

 violet effects and in photoreactivation are very imperfectly understood, 

 and the observations that have been reported for various organisms can- 

 not yet be discussed from a unified point of view. Ultraviolet can affect 

 any substances which absorb the radiation. To this class of substances 

 belong the nucleic acids, the proteins, and numerous smaller molecules 

 with specific functions. There have been no reports of photoreactivation 

 of enzymes inactivated by ultraviolet; however, the following significant 

 observation has been published by Shu gar (1951). Crystalline triose- 

 phosphate dehydrogenase, which is firmly combined with diphosphopyri- 

 dine nucleotide (DPN), is partially inactivated during preparation, owing 

 to oxidation of essential SH groups, and can be reactivated by exposure to 

 light of the near-ultraviolet spectrum, with a peak at 3400 A, which is 

 therefore in the absorption band of reduced DPN. The mechanism of 

 reactivation could be a reduction of S — S groups consequent to light 

 absorption in contiguous reduced DPN molecules. Here are found 

 several features which occur in photoreactivation: the spectral range 

 involved is similar; only a fraction of the enzyme activity can be reac- 



