34 MICROSOMAL PARTICLES 



phosphate buffer the optical density rises at once as the temperature is raised 

 above room temperature and reaches a maximum value, 28 per cent higher, at 

 about 80° C. At higher ionic strength the rise is similar but does not begin 

 until a higher temperature is reached. At lower ionic strength the optical 

 density at room temperature is already considerably above the lower limiting 

 value. Thus at the reduced ionic strength, where viscosity measurements show 

 that the molecule is partly expanded, these measurements indicate that the 

 hydrogen bonding is correspondingly reduced. These observations are consist- 

 ent with the initial hypothesis and support the view that the intrachain attrac- 

 tions in RNA arise from hydrogen bonding between the purine and pyrimidine 

 bases. The pairing would presumably be similar to that existing in DNA, but 

 this fact carries no implication that the base pairs would be periodically 

 organized. 



Since the magnitude of the hypochromic effect (about 28 per cent) is more 

 than half the total that results from the hydrolysis of RNA [9], the implica- 

 tion is that a large fraction of the purine and pyrimidines participate in the 

 pairing when the ionic strength is 0.01 M or more. The fraction of base pairs 

 involved appears to be unchanged by the heating cycle. Indeed, the first heat- 

 ing cycle of freshly prepared RNA shows the same results as successive cycles, 

 in contrast with DNA, where the optical density never returns to the original 

 value after the first heating. One would therefore conclude that there is no 

 periodic arrangement of base pairs in RNA as there is in DNA. 



Finally, it is of interest to note that the heat treatment used to stabilize the 

 RNA, heating to 83° C of a 0.01 M phosphate buffer solution, is precisely the 

 treatment required to reach the maximal optical density and presumably break 

 the hydrogen bonds. Consequently the drop in apparent molecular weight pro- 

 duced by the heat treatment may have done nothing other than permit some 

 aggregates of RNA molecules to be dissociated through the opening-up of the 

 hydrogen bonds holding them together. It remains for future work to show 

 whether or not this interchain bonding is a remnant of structural organization 

 of RNA in the microsomal particle. 



ACKNOWLEDGMENT 



We are deeply indebted to Dr. Norman S. Simmons, Atomic Energy Project, 

 University of California at Los Angeles, for his helpful advice and discussions 

 regarding the preparation of RNA. One of us wishes to thank also the Na- 

 tional Science Foundation and the Union Carbon and Carbide Corporation for 

 fellowship support during the course of this investigation. This investigation 

 was supported by the National Institutes of Health (C2170) . 



Note Added in Proof 



Using aqueous phenol for removal of protein, we have isolated from calf-liver 

 microsomal particles RNA of substantially higher molecular weight (~1 X 10°) 

 than that obtained in detergent RNA preparations. This large RNA appears 



