102 R. MARKHAM 



1. Points of Similarity 



The nucleoprotein (bottom) and protein (top) components resemble each 

 other in many ways. They each contain about 3 X 10^ of protein (as molecular 

 weight), and the amino acid compositions of these proteins are closely 

 similar (Markham ei al., 1948a; Eraser and Cosentino, 1957). The two proteins 

 electrophorese together, having isoelectric points of pH 3.75, they both 

 crystallize as octahedra from salt solutions, and they will form mixed crystals. 

 The diffusion coefl&cients are very similar, and the diameters of the hydrated 

 particles are very closely alike (about 280 A), as shown by the diffusion 

 measurements. X-ray crystallography (Bernal and Carlisle, 1948), and X-ray 

 scattering. The particles are essentially spheres as seen on the electron 

 microscope. Both types of particle react with antiserum to the virus, and no 

 differences can be detected by serological means. 



2. Points of Difference 



The nucleoprotein contains about 37 % of ribonucleic acid (as the free 

 acid) and has an ultraviolet absorption maximum at 263 m/x. The optical 

 density of a 1-cm. layer at 0.1 mg./ml. is 0.95. The protein component has 

 an ultraviolet absorption maximum at 276 m/x, where the density of a 

 O.l-mg./ml. solution is 0.11. Although the crystals are similar in appearance, 

 (Fig. 16), the crystals of the protein component float when centrifuged in a 

 sodium thiosulfate solution of specific gravity 1.32, while the nucleoprotein 

 crystals sink. The nucleoprotein has 3.77 % of phosphorus, while the protein 

 has none. The specific volume of the nucleoprotein is 0.67, and that of the 

 protein is 0.74. The molecular weights are 5 X 10^ and 3 X 10^, respectively 

 (Markham, 1951). The nucleoprotein is a much better antigen (inducing 

 antibody formation). X-ray scattering shows that the nucleoprotein is 

 essentially solid, and that the protein is hoUow with an internal diameter 

 of about 75 % of the external one (Schmidt et al., 1954). The protein com- 

 ponent also tends to coUapse more on drying for electron microscopy 

 (Cosentino et al., 1956). 



3. The Interpretation of the Above 



By a study of the above it was easily seen in what way the two materials 

 were related. The nucleoprotein component quite evidently consists of a sheU 

 of protein which encloses a core containing the ribonucleic acid. The ribo- 

 nucleic acid is necessary for the infectivity of the virus particles, but it does 

 not have much effect on the surface properties of the virus, which are 

 essentially decided by the amino acid side chains on the protein surface. 



The exact fmiction of the "top" component is uncertain and considerable 

 speculation has been wasted on the problem. One of the striking factors is 

 that the top component is always present in every host, and the proportion 



