4 MICROSOMAL PARTICLES 



and, with method I, some Carborundum that was not removed at lower speeds. 

 The supernatant liquid from this step is now centrifuged at 105,00% for 60 

 minutes. The pellet so obtained is solubilized in RNP buffer for approximately 

 12 hours, and then centrifuged at 8700g for 15 minutes. The precipitate is dis- 

 carded, and the supernatant liquid is examined in the analytical ultracentrifuge 

 to determine the number of sedimenting components, their relative amounts, 

 and their sedimentation coefficients. When the 86 S component is desired, the 

 105,000^- and the 8700^ cycle is repeated until over 90 per cent of the area in the 

 schlieren pattern is under the appropriate peak. 



PROPERTIES OF RIBONUCLEOPROTEIN PARTICLES 



When a crude extract is processed to the stage labeled "preparation" in 

 figure 1, and is examined in the analytical ultracentrifuge, it is found to sedi- 

 ment as a single peak of sedimentation coefficient 86 S. If, however, the same 

 crude extract is carried to the same stage employing a buffer in which the mag- 

 nesium concentration has been reduced to 10~ 3 M, the ultracentrifuge pattern 

 now shows five significant components. Comparison of the schlieren and ultra- 

 violet absorption photographs in the ultracentrifuge suggests that all these com- 

 ponents contain nucleoprotein. The sedimentation coefficients extrapolated to 

 zero concentration and corrected to 20° C are 86, 77, 58, 39, and 10 S. The 86, 

 58, and 39 S components are usually found in largest amount. Figures 2 and 3 



Fig. 2. An electron micrograph of the edge of a droplet of RNP particles sprayed onto 

 a collodion membrane and shadowed with uranium. Magnified 34,000 times. Taken on a 

 Siemens Elmiskop I by Professor Paul Kaesbcrg. 



