266 



HEINZ SCHUSTER 



TABLE VI 



Alteration of the Ultraviolet Absorption on 



Degradation of TMV-RNA 



Substrate 



Treatment 



Increase in 

 UV absorp- 

 tion at 

 260 m M 



(%) 



TMV-RNA 

 TMV-RNA 

 TMV-RNA 

 TMV-RNA "core' 



1 M NaOH, 8 hours, 37°C. 32 



Leaf ribonuclease, 8 hours, 37 C. 37 



Pancreatic ribonuclease 12 hours, 37°C. 15 



1 M NaOH or leaf ribonuclease, 12 hours, 37°C. 21 



sedimentation constant. The relationship between [77] and s for RNA lies 

 somewhat between these two extremes, suggesting that RNA in solution is 

 a flexible structure, intermediate between a random coil and a rigid rod. 



UV absorption. The sum of the absorption at 260 m/i of the component 

 nucleotides of an RNA molecule is higher than the absorption of the un- 

 degraded RNA molecule. This so-called hyperchromic effect can be strik- 

 ingly demonstrated with TMV-RNA. 6874 The increase in absorption of 

 TMV-RNA after various treatments is presented in Table VI. The RNA 

 was obtained, in this case, by heat denaturation of TMV. Alkali degrada- 

 tion or treatment with leaf ribonuclease causes a similar increase in UV 

 absorption. This is not unexpected since leaf ribonuclease disrupts all bonds 

 between component nucleotides of RNA, in contrast to pancreatic ribo- 

 nuclease. 75 Indeed, the increase in UV absorption obtained after incubation 

 of TMV-RNA with pancreatic ribonuclease is less than half that obtained 

 after alkali degradation. 



The results with RNA obtained by heat denaturation are quite different 

 from those using RNA extracted with phenol. After alkali degradation of 

 the latter, a 30 % increase in UV absorption was observed, as was also true 

 in the former case. 76 But after incubation of phenol-extracted RNA with 

 pancreatic ribonuclease for only ^ hour, the increase in UV absorption 

 was more than 25% (see Fig. 5). Gierer 68 concluded from this observation 

 that the change to higher absorption must occur when the average poly- 

 nucleotide size has decreased to about 20 nucleotides; therefore, the effect 

 may be connected with the structure of larger RNA molecules in solution. 



If the residue resistant to pancreatic ribonuclease ("core") is treated with NaOH 

 or leaf ribonuclease, there is an increase in absorption of 20% which accounts for 60% 



74 K. K. Reddi, Biochim. et Biophys. Acta 27, 1 (1958). 



75 K. K. Reddi, Biochim. et Biophys. Acta 28, 386 (1958). 



76 H. Schuster, unpublished work (1958). 



