The Biochemistry of Lysogeny 53 



Table 1. The Inducing Effeet of Irradiated Leueovorin on E. coli K,., 



Num- 

 ber Experiment 



1 E. coli K-12 control 



2 Exposed to 15 seconds direct irradiation 



3 Incubated with 500 jig/ml. of irradiated 



leueovorin 



4 Incubated with 500 pg/ml. of irradiated 



2-amino-4-hydroxy-5-formyl-6-methyl- 

 5,6,7,8-tetrahydropteridine 



be achieved, because the photolytic products, unlike their unirradiated 

 precursors, inhibit, above a concentration of 500 micrograms per 

 milliliter, the bacterial respiration essential for phage development. 

 Whether induction approximating the direct irradiation of the bacteria 

 can be obtained with the photolytic products will depend upon whether 

 a separation of the inducing and inhibitory potencies can be effected. 



The induction by the photolytic products, unlike the mutagenesis by 

 irradiated complex media reported by Wyss 18 and collaborators, is 

 not caused by accumulated hydrogen peroxide. The concentration of 

 hydrogen peroxide after irradiation in the inducing solutions is too low- 

 by a factor of 10 to affect E. coli K 12 . Moreover, induction by hydro- 

 gen peroxide is, of course, completely suppressed by catalase, whereas 

 solutions of the photolytic products of leueovorin are completely un- 

 affected by this enzyme. 



Since attempts at the identification of the inducer are just starting, 

 one can speculate freely on its nature, untrammeled by the intrusion 

 of obstreperous facts. An organic peroxide is one possibility, since 

 some peroxides are known to act as inducers in lysogenic organisms. 

 The sequence of reactions during the photolysis of folic acid is well 

 known: cleavage occurs at the 6-methylene link, the methylene carbon 

 being oxidized to an aldehyde, yielding 2-amino-4-hydroxy-6-formyl- 

 pteridine. On further irradiation this compound is oxidized to the 

 corresponding 6-carboxylic acid. 17 



Leueovorin differs from folic acid in containing a reduced ring system 

 and a formyl group in the 5 position. By, perhaps oversimplified 

 analogy, one might expect both the 5 and 6 carbons in leueovorin to 

 be involved in oxidation, possibly yielding a peroxide. At any rate, 

 the presence of the formyl group of leueovorin is essential, for the 

 reduced form of folic acid, tetrahydro folic acid, does not yield an 

 inducer on irradiation; nor does 10-formyl folic acid. 



However, if the inducer is a peroxide it is an unusually stable one; 



