RIBONUCLEIC ACIDS 



this time that most of the hnkages in RNA are of the 3 ',5' (or 

 2 ',5') variety, and definitely 3 ',5' in all specimens so far tested 

 by the spleen enzyme referred to earlier (8,29,30,63). 



The implications of this structure are many. The similarity 

 in backbone revives speculation as to whether or not there is an 

 interconversion of RNA and DNA ; only a reduction at the C2 

 carbon of ribose (and the methylation of uracil) is necessary to 

 convert one into the other. The spatial considerations which 

 fortify the consideration of DNA as a template for protein (35) 

 now seem applicable to RNA ; in the absence of definite knowl- 

 edge as to which precedes, RNA, DNA, and protein may all be 

 considered as templates of one another. Then there is the 

 possible relation of the 5' mononucleotides which exist in 

 cytoplasm to consider. 



When the 5' linkage was first demonstrated in RNA, only 

 two varieties of 5 ' nucleotides had been identified in tissues : ( 7) 

 adenosine 5 '-phosphate, which occurs alone and in pyro- 

 phosphate combination in adenosine triphosphate, di- and 

 tri-phosphopyridine nucleotide, flavinadenine dinucleotide, and 

 coenzyme A; (2) uridine 5 '-phosphate, identified in several 

 pyrophosphate combinations in penicillin-treated bacteria (51), 

 and as part of a coenzyme (12). Within the past three years, and 

 at least partially because of the extension of the same ion- 

 exchange chromatographic technique to the analysis of the 

 acid-soluble phosphates of tissues, it has become apparent that 

 the 5 '-phosphates, -diphosphates, and -triphosphates (all in 

 pyro linkage) of all four ribonucleosides exist generally in tissues 

 (33,58,59). Whereas some of the diphosphates are linked 

 through the pyrophosphate group to another phosphate, as in 

 adenosine triphosphate, some are combined with nonnucleotide 

 groups; examples of this type are now known for cytidine 

 (3,38), uridine (12,13,21,33,58-61), and guanosine (11,50), 

 some having been discovered by ion-exchange chromatography 

 (3,11,33,59). Enzyme-catalyzed equilibria among the tri- 

 phosphates as well as coenzyme functions of some of these 

 are now known to exist (53,65). The recent demonstra- 



469 



