EXPERIMENTS ON PHOTOREACTIVATION OF BACTERIOPHAGES 

 INACTIVATED WITH ULTRAVIOLET RADIATION^ 



R. DULBECCO 



Department of Bacteriology, Indiana University, Bloomington, Indiana'^ 



Received for publication October 24, 1949 



Kelner (1949), working \\\ih. conidia of Streptoniyccs griseus, discovered that 

 light belonging to the visible range is capable of reactivating biological ma- 

 terial that has been rendered inactive by ultraviolet radiation (UV). Shortly 

 after Reiner's discovery was known, a similar phenomenon in bacteriophages 

 (bacterial viruses) was observed by accident. Plates of nutrient agar containing 

 UV-inactivated phage and sensitive bacteria had been left for several hours on 

 a table illuminated by a fluorescent lamp. After incubation it was noticed that 

 the number of plaques was higher on these plates than on similar plates incu- 

 bated in darkness. A short report of this phenomenon of "photoreactivation" 

 (PHTR) has already been published (Dulbecco, 1949). The present paper con- 

 tains the results of a first group of experiments concerning PHTR of seven bac- 

 teriophages of the T group active on Escherichia coli, strain B. 



MATERIALS AND METHODS 



Stocks of each phage were prepared by inoculating material from a single 

 plaque into a culture of E. coli B in a synthetic medium M9,^ except for phage 

 T5, of which a stock in Difco nutrient broth was used. In some experiments the 

 phage was purified by two or three steps of differential centrif ugation ; the phage 

 was resuspended in m/15 phosphate buffer pH 7, with MgS04 added to a con- 

 centration 10-2 M. Unless otherwise specified, the experiments described in this 

 paper were performed with phage T2. Escherichia coli, strain B, was used through- 

 out. In some experiments bacteria were grown in nutrient broth with aeration 

 and the culture was infected with phage when it was in the logarithmic phase 

 of growth (about 10^ cells per ml); these bacteria will be referred to as "bac- 

 teria in broth." In other experiments bacteria were grown in broth up to a con- 

 centration of about 2 X 10* cells per ml, then washed with saline (0.85 per cent 

 NaCl) and resuspended in saline, kept at 37 C for 30 minutes, and then infected; 

 these bacteria will be referred to as "resting bacteria." 



1 This work was done under an American Cancer Society grant, recommended by the 

 Committee on Growth of the National Research Council, under the direction of Dr. S. E. 

 Luria. The author wishes to express his appreciation to Dr. Luria for facilitating this work 

 materially and for numerous discussions during its progress. The manuscript was completed 

 at the California Institute of Technology. The author also wishes to acknowledge his in- 

 debtedness to Dr. M. Delbriick for helpful discussions on the interpretation of the data. 



2 Present address : Kerckhoff Laboratories of Biology, California Institute of Technology, 

 Pasadena 4, California. 



3 NH4C1,1.0 g; KH2PO4, 3.0 g; NaoHPO4,6.0 g; NaCl, 0.5 g; MgS04, 0.1 g; distilled water, 

 1,000 ml; 4 g per liter glucose added after separate sterilization. 



Reprinted by permission of the author and the Williams and 



Wilkins Co., from the Journal of Bacteriology, 59 (3), 329-347, 



March, 1950. 



228 



