14 MOLECULES, VIRUSES, AND BACTERLV 



hydrogen bonding between the two, does the transfer RNA actually 

 polymerize in the ribosomes, and with which component is the amino 

 acid associated before it enters peptide linkage? 



It has frequently been noted that transfer RNA is of an unwieldy 

 size for carrying out its adaptor role. Why should nature require so 

 many nucleotides to specify the locus of one amino acid in a peptide 

 chain? One appealing suggestion is that nature may require consider- 

 able redundancy of information in order to avoid errors (Yoshikawa, 

 1960). Were transfer RNA of too small a size, it would be unusually 

 vulnerable to mutational change. This would be an intolerable situ- 

 ation, for transfer RNA deals only with amino acids, and a mutation 

 in one of its species would affect the synthesis of all protein containing 

 that amino acid. By requiring that many nucleotides specify the amino- 

 acid locus, nature reduces the danger resulting from a mutational 

 change of a single nucleotide. 



It is clear that more questions can be raised than can be answered. 

 It is also clear, however, that biology has advanced beyond the descrip- 

 tive stage and that, at the molecular level, we may formulate hypoth- 

 eses and subject them to increasingly critical experimental tests. 



Note Added in Proof. The statements made in the foregoing require 

 modification in the light of a series of new discoveries which suggest 

 that ribosomes themselves are able to synthesize protein only in the 

 presence of a "messenger RNA." This RNA is presumed to be a product 

 of a structural gene determining the amino-acid sequence of a given 

 protein. The messenger enters the cytoplasm and lays out its base se- 

 quence pattern on a ribosome, thereby conferring specificity on the 

 latter. It is upon this messenger template that the protein is then made. 

 Many of the observations are consistent with the postulate that the 

 messenger has a brief functional period of existence. Thus a dynamic 

 control is exerted by DNA over the process of protein synthesis. 



We may briefly summarize the evidence supporting this new hy- 

 pothesis. ( 1 ) It has long been observed that the base composition of the 

 ribosomal RNA of a given species frequently does not reflect the base 

 composition of its DNA. If most of the ribosomal RNA were involved 

 in template function, one might expect a correlation between its struc- 

 ture and that of the DNA from the same source. ( 2 ) Upon phage in- 

 fection of bacteria, there is a cessation of synthesis of bacterial ( ribo- 

 somal ) RNA, in spite of a rapid synthesis of new phage-specific protein. 

 Concomitantly there is a sharp rise in turnover rate of an RNA com- 

 ponent, existing in low concentration, having sedimentation properties 

 different from either transfer RNA or ribosomes. This special RNA 

 fraction reflects the base compositions of the invading phage DNA. It 

 can also be shown, under appropriate conditions, to be reversibly asso- 

 ciated with the ribosomes of the infected bacteria. (3) Numerous ex- 



