262 HEINZ SCHUSTER 



versely related to s 2 . The viscosity corresponding to a component of sedi- 

 mentation constant s A may be obtained from the equation 



[vU/b] = May* (i) 



It was found, experimentally, that the sedimentation constant of component 

 A(s A ) is 1.4 times as large as the average s, the sedimentation constant of 

 the entire RNA preparation. Thus, a viscosity 1.8 times as large as the 

 average viscosity of components A and B corresponds to the viscosity of 

 the infectious molecule. From this empirical relationship, it was possible to 

 calculate a molecular weight of 2 X 10 6 for the infectious RNA. 



(2) Determination of Molecular Weight from Kinetics of Degradation with 

 Ribonuclease. From the correlation between decrease in molecular weight and 

 infectivity due to the action of ribonuclease. The average molecular weight of 

 an RNA preparation is approximately proportional to its viscosity. The 

 infectivity of the RNA is approximately proportional to the number of 

 local lesions which appear on leaves of Nicotiana glutinosa plants upon 

 infection with RNA. A typical example of the measured decrease in in- 

 fectivity (7), viscosity [rj], and mean molecular weight (m) is given in 

 Table V. The decrease in I is much faster than that in m. This indicates 

 that the molecular weight of the infectious component (M) is larger than 

 m. Using the quantitative relationship derived by Gierer 11 



1/im - l/m, = jjjj 0* h ~ ln T ^ &) 



it is possible to calculate a molecular weight of 2.2 X 10 6 employing the 

 values given in Table V. 



From the correlation of decrease in infectivity with the splitting activity of 



TABLE V 

 The Alteration of Infectious TMV-RNA by Treatment with Ribonuclease 



t (minutes) [17] m I (lesions) 



a Decrease of intrinsic viscosity, [i\], mean molecular weight (weight average) m, and infectivity, /, with 

 time t. The decrease was induced by 1.5 X 10 _J Aig./ml. ribonuclease at 22°C. in a solution of 0.16% ribonucleic 

 acid in 0.02 M phosphate buffer, pH 7. The infectivity, /, is given as the number of lesions per 25 leaves of 

 Nicotiana glutinosa, inoculated with RNA. [Taken from A. Gierer, Nature 179, 1297 (1957); Z. Naturforsch. 

 13b, 485 (1958).] 



