37. NUCLEIC ACID AND PROTEIN SYNTHESIS 



359 



TABLE II 

 Amino Acid (AA) Composition of Microsomal Particles and of Plant Viruses" 



a P. O. P. Ts'o, J. Bonner, and H. Dintzis, Arch. Biochem. Biophys. 76, 225 (1958). 



18 *S in a Mg ++ -free buffer. Whether these large molecules have covalent 

 continuity is not clear, however, although they were analyzed in a Mg ++ - 

 free medium and their sedimentation properties were unaltered after re- 

 processing by the detergent procedure. 



Another parameter of the particle which shows constancy among differ- 

 ent species is the amino acid composition of its protein 51 (Table II). As 

 will be mentioned later, in vivo labeling of particles by amino acids has 

 revealed that only a very small fraction of the total protein of the particles 

 is actually in the process of synthesis. The remainder is presumably struc- 

 tural and therefore might be expected to be constant in composition over a 

 wide phylogenetic range. This is suggested by the preliminary data. Note 

 the absence of cysteine in this protein, an observation also made by Roberts 

 et a/. 16d Whether this structural protein is made up of subunits of a single 

 molecular species or consists of several proteins is unknown. Roberts et 

 al. 16d have obtained evidence for considerable heterogeneity of E. coli 



51 P. O. P. Ts'o, J. Bonner, and H. Dintzis, Arch. Biochem. Biophys. 76, 225 (1958). 



