88 D. SHUGAR 



acid, in which hyperchromicity is too small to raise any doubts as to the 

 validity of the findings. Only a brief outline of these results, which sub- 

 stantiate and considerably extend those outlined above, will be given here. 69 



The rate of photolysis of UpUp corresponds to a reaction order of about 2.5 up 

 to the point where about 50% of the aromatic rings have reacted, following which it 

 drops to nearly unity; the initial quantum yield is about twice that prevailing for 

 an equal concentration of Up alone. The extent to which the reaction may be reversed 

 in the dark is, however, dependent on the degree of photolysis. Up to the point where 

 50% of the uracil rings have reacted, only 50% of the loss in absorption can be restored 

 by heat; further decreases in absorption as a result of irradiation are completely re- 

 versible, so that total thermal reactivation may attain over 75%. 



It follows from the above that in a dinucleotide, one of the rings will react more 

 readily than in the free state but that if the second ring has not reacted, the chances 

 of reversal for the photolysed ring are considerably reduced. Hence despite the prac- 

 tical absence of hyperchromicity in poly-U (Section IV, 4) it must be concluded that 

 interaction does prevail between the aromatic rings. That this is not due simply to 

 incorporation in the chain, as suggested by Warner, 72 is shown by the fact that the 

 quantum yield for uridine-2'(3') ,5'-diphosphate is the same as for other uridylic 

 acid isomers and the degree of thermal reversibility over 90% (Table II). 



The behavior of UpU and UpUp! is entirely analogous to that for UpUp, as is 

 also that of the isomer of UpUp 79 in which the internucleotide linkage is 2', 5'. 



The behavior of longer chains is similar qualitatively; when we get to highly poly- 

 merized poly-U the maximum extent of thermal reactivation following photolysis 

 drops to about 55%, as compared to over 75% for a dinucleotide. 



It can therefore be considered as reasonably well established that reversible photo- 

 decomposition of free pyrimidine nucleotide residues is duplicated when these are 

 incorporated in polynucleotide chains. It remains to establish what has happened to 

 that portion of the residues which apparently does not undergo reversibility, i.e., 

 whether they have been irreversibly photolysed or whether adjacent unaltered rings 

 inhibit reversal. 



The desirability of extending the above results to poly-C as well as copolymers of 

 pyrimidine and purine nucleotides, particularly di- and trinucleotides, is obvious 

 (see Addendum). 



2. Biological Studies 



It has long been known that ultraviolet inactivated bacteriophage can- 

 not be photoreactivated outside of their host cells, a fact suggestive of the 

 involvement of some cellular factor or mechanism. 181 Attempts have been 

 made to photoreactivate ultraviolet-inactivated transforming DNA with 

 near ultraviolet light, but without success. 171 ' m It would indeed have been 

 surprising if the results had been positive since DNA does not exhibit 

 specific absorption in the near ultraviolet. 



No trials as yet appear to have been made to reactivate ultraviolet in- 

 activated transforming DNA by heating at temperatures which do not 

 normally result in heat inactivation, although this would be well worth 

 trying. 



181 J. Jagger, Bacleriol. Revs. 22, 99 (1958). 



