37. NUCLEIC ACID AND PROTEIN SYNTHESIS 371 



that intrinsic protein synthetic activity in nuclei, chloroplasts, and mito- 

 chondria is not due to ribosome-like particles which might be an integral 

 part of these structures. These matters will be discussed further below. 



d. Comment on the Metabolic Origin of Ribosomes 



Before concluding this section it might be pertinent to ask : how do ribo- 

 somes arise? What machinery is responsible for the synthesis of their RNA 

 and protein? There is still no clear answer to this question but there is a 

 growing body of evidence indicating that the nucleus is capable of carrying 

 out both RNA and protein synthesis, that particles very similar to those 

 we have been describing are found in the nucleus, that cytoplasmic RNA 

 cannot be reconstituted once it has been removed by ribonuclease action 

 unless a nucleus is present, and that cytoplasmic RNA (or its precursors) 

 moves from the nucleus to the cytoplasm but not in the reverse direction. 

 These observations point strongly toward the nucleus as the likely site of 

 ribosome synthesis. Detailed discussion of the evidence in support of these 

 statements may be found in the reviews of Brachet, 1 Bonner, 12 and Chan- 

 trenne 8 in particular. 



2. The Soluble Enzyme-Transfer Ribonucleic Acid System 



Developments in understanding the soluble requirements in the cell-free 

 mammalian amino acid incorporation system illuminated another, and more 

 direct, participation of RNA in protein synthesis (see Fig. 1). It was learned 

 that the soluble, nonparticulate liver cell fraction (earlier referred to as the 

 "pH 5 fraction") found necessary for the over -all incorporation of amino 

 acids into ribosome protein contained enzymes which catalyzed the car- 

 boxyl activation of amino acids by formation of enzyme-bound amino acyl 

 adenylate compounds from ATP and amino acids, eliminating PP as a 

 product. 112, 113 Similar enzymes were found in yeast and bacterial prepara- 

 tions 114 ' 115 and have since been found in all tissues examined (cf. Novelli 

 and De Moss 3 and Hoagland 14 ). Although it was gratifying to find amino 

 acid activation reactions in a cellular fraction known to be required for 

 amino acid incorporation into protein, a causal connection between the two 

 processes was not established. Of considerably more pertinence to the sub- 

 ject of this chapter, however, was the finding that this same soluble cell 

 fraction contained a species of RNA uniquely capable of reaction with amino 



1,2 M. B. Hoagland, Biochim. et Biophys. Acta 16, 288 (1955). 



113 M. B. Hoagland, E. B. Keller, and P. C. Zamecnik, J. Biol. Chem. 221, 45 (1956). 



114 P. Berg, J. Biol. Chem. 222, 1025 (1956). 



115 J. A. DeMoss and G. D. Novelli, Biochim. et Biophys. Acta 22, 49 (1956). 



