448 F. GROS 



the growth of E. coli. The rate of this renewal depends to a large extent on 

 the capacity of the cell to synthesize protein. Thus, the bound amino acids 

 are actually utilized for the formation of protein molecules. 



(3) Direct transfer of radioactivity can be observed in vivo il (as well as 

 in vitro 30 ) from sRNA labeled with radioactive amino acids, to protein 

 fractions. 



There is however a possibility that other pathways than amino acid 

 activation and transfer to sRNA exist, either for the formation of peptide 

 linkages in general, or at least for the synthesis of part of the cellular pro- 

 tein. The protein of the membrane or the basic protein of the nucleoprotein 

 particles could belong to this last category. This restriction to the general 

 scheme previously described, is suggested by the results of various experi- 

 ments with subcellular fractions of bacteria (cf. Chapter 37 by Hoagland 

 on protein synthesis in "in vitro systems"). 



2. Nucleic Acid as Constituents of the Protein-Forming System 



The sRNA cannot be considered as a template for the formation of pro- 

 tein owing to its small size (200 nucleotides), nor does it function as a tem- 

 plate for the synthesis of peptides, since it is known that in vivo, as in vitro, 

 only one single molecule of amino acid is attached per molecule of 

 sRNA, 39 • 41 and also since the possibility of peptides acting as protein pre- 

 cursors is excluded by several experiments. Therefore the amino acid se- 

 quence has to be determined by templates of much higher molecular weight, 

 and the role of the sRNA can be visualized rather as that of a "steric adap- 

 tor" of the amino acids to specific loci of this template. 165 Several now well- 

 known hypotheses predict how the base sequence of highly polymerized 

 nucleic acid can "code" the amino acid sequence of the proteins. The criti- 

 cal question is the nature of the nucleic acids which most probably serve 

 as direct template for the synthesis of protein. Put in more precise terms, 

 what are the respective roles of DNA and RNA in protein synthesis? 



It is obvious from genetic evidence that the potentiality for a cell to make 

 a specific protein is located in very precise regions of the DNA molecules. 

 Therefore, the simplest hypothesis concerning the mechanism of protein 

 synthesis could be that DNA serves directly as a template for the assembly 

 of free or activated amino acids into a specifically ordered polypeptide 

 chain. 199a 



i99a Thg recen t observation by Pardee, Jacob, and Monod 140 on enzyme synthesis 

 during genetic recombination in E. coli K12 is relevant to this hypothesis. When 

 a strain Hfr injects its lac + gene into the cytoplasm of an F~~ strain, which is lac - , 

 jS-galactosidase synthesis starts immediately in the zygote and with its maximal 

 rate. Formation of the new protein does not appear to require a macromolecular 

 replicate of the DNA; or at least this replicate would be formed only once by DNA 

 and very rapidly. 



