370 MAHLON B. HOAGLAND 



of the protein originally present) during prolonged incubation in phosphate 

 buffer and Mg ++ . 110 The extent of incorporation of phenylalanine-C 14 into 

 protein in this system coincides with the amount of new protein formed 

 The newly formed material gives a positive test with the Lowry reagent 

 and is precipitable by 5 % trichloroacetic acid. The particle preparation is 

 complex, however, apparently containing indigenous activating enzymes, 

 ATP, and amino acids. Perhaps for this reason, it has not been possible 

 thus far, to show a requirement for these adjuvants for the synthesis ob- 

 served. 



Webster 111 also states preliminarily that pea seedling particles will effect 

 a net synthesis of a considerable quantity of protein, such that it can be 

 measured gravimetrically. This system is said to be dependent on addition 

 of amino acid activating enzymes, ATP, GTP, a full complement of amino 

 acids, and an "unidentified fraction," thought to be polynucleotide in na- 

 ture. 



We may ask before concluding this section: what is the minimal size of 

 ribosome capable of incorporating amino acids into its protein? Very little 

 work has been done on this subject in the era since control of particle dis- 

 sociation by Mg + + has been understood, but it would appear from wisps of 

 evidence that ribosomes smaller than the intact 80 S particles are able to 

 incorporate amino acids. Thus Littlefield and Keller, 35 and Simkin and 

 Work, 48 using ribonucleoprotein particles from ascites tumor cells, prepared 

 by treatment of cell lysates with 0.5 M NaCl, obtained activity in particles 

 with sedimentation constants as low as 20 S. Sachs has obtained incorpora- 

 tion in ribonucleoprotein material obtained from rat liver by pyrophosphate 

 treatment (a measure which apparently functions to lower the effective 

 Mg ++ concentration) in which there was dissociation, though of undeter- 

 mined extent. 103 ' 104 Siekevitz and Palade 40 find that a poorly sedimenting 

 small particle fraction has protein-synthetic activity. Webster 61 has treated 

 pea seedling particles by sonic vibration and found that the resulting non- 

 sedimentable material (about 40 %) was better able to incorporate glutamic 

 acid into protein than the original particles. However, it should now be pos- 

 sible to repeat this kind of study under carefully controlled conditions to 

 correlate incorporating ability and particle size. 



It would seem reasonable, therefore, to conclude that ribosomes play a 

 direct and major role in protein synthesis in most tissues. Whether or not 

 they can account for all protein synthesis is certainly not known, and 

 evidence that other cellular particulates carry out autonomous protein syn- 

 thesis leads us to be cautious. It has not been convincingly shown, however, 



110 I. D. Raacke, Biochim. et Biophys. Acta 34, 1 (1959). 



111 G. E. Webster, Federation Proc. 18, 1379 (1959). 



