37. NUCLEIC ACID AND PROTEIN SYNTHESIS 3G9 



TABLE IV 



Incorporation of C 14 -Amino Acids into Rabbit Reticulocyte Ribosomes 



Compared with Amino Acid Composition of Rabbit Globin 



Complete system" 



Leucine-C 14 



Isoleucine-C 14 



Valine-C 14 



a The complete system contained rabbit reticulocyte ribosomes, guinea pig pH 5 fraction, creatine kinase, 

 creatine phosphate, ATP, GTP, and a complete amino acid mixture. Omission of any one of these components 

 reduced the activity to one-half or more. Data are from R. Schweet, H. Lamfrom, and B. Allen, Proc. Natl. 

 Acad. Sci. U. S. 44, 1029 (1958). 



phase could be reproduced in a cell-free system using ribonucleoprotein par- 

 ticles previously labeled in whole reticulocytes which were then incubated 

 with unlabeled soluble fraction, phosphogly cerate, ATP and Mg ++ . This 

 kind of observation has been amplified and extended by Schweet. He has 

 shown that ribonucleoprotein particles from reticulocytes will synthesize 

 hemoglobin, or a molecule very like it, in vitro. m Schweet showed that in- 

 corporation of labeled amino acids into hemoglobin in a cell-free preparation 

 derived from reticulocytes required ribonucleoprotein particles, ATP and an 

 ATP-generating system, and a soluble fraction containing amino acid ac- 

 tivating enzymes. The radioactivity initially appeared in the protein of the 

 particles, but upon further incubation could be shown to appear in soluble 

 protein characterized as hemoglobin by isolation with carrier unlabeled 

 hemoglobin. Using three labeled amino acids it was further shown (see Table 

 IV) that the radioactive protein in the particles contained these amino acids 

 in a ratio characteristic of hemoglobin, and not characteristic of the struc- 

 tural protein of the particles. Furthermore, synthesis of hemoglobin in this 

 cell-free system was possible only in the presence of a mixture of amino acids 

 in the proper proportions. These proportions were those shown by Borsook 

 et al. 109 to be optimal for the synthesis of hemoglobin by intact reticulocytes. 

 It seems, therefore, highly probable that new hemoglobin molecules or mole- 

 cules very closely resembling hemoglobin were synthesized in the particles 

 and subsequently released into the soluble milieu. 



Raacke reports that ribonucleoprotein particles isolated from pea seed- 

 lings will effect a net increase in protein-like material (approximately 10 % 



108 R. Schweet, H. Lamfrom, and E. Allen, Proc. Natl. Acad. Sci. U. S. 44, 1029 (1958). 



109 H. Borsook, E. H. Fischer, and G. Keighley, J. Biol. Chem. 229, 1059, 1957. 



