STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 199 



a. Lipmann (1954) initially proposed an activation of specific sites of some 

 unspecified template structure by transfer of the pyrophosphoryl group from 

 ATP. This exchanged with the carboxyl of an amino acid whose nature was 

 determined by the specific surface. In a later step, the bound carboxyl- 

 activated amino acids spontaneously condensed to form a polypeptide chain, 

 freemg the surface for a renewed activation, 



b. Borsook (1956) proposed that the amino acids were linked via carboxyl 

 groups to the phosphates of the nucleic acid. The amino acids were then con- 

 densed to polypeptide by the action of some enzyme. In the similar mechan- 

 ism of Novelli and DeMoss (1957), which takes into account the known data 

 on initial amino acid activation and subsequent transport of bound inter- 

 mediates, it is suggested that the bound amino acids spontaneously condense 

 when the template is fully charged or a mechanism exists for undirectional 

 condensation (Dalgleish, 1957). Spontaneous condensation is facilitated by 

 the proximity (2. 2 A) of a carboxyl group of one amnio acid to the amino 

 group of another and is presented in formula (XXXV). 



-^ RNA 

 4- 

 O 



HOOC-CH^NHC-CH^NHj 



+ Amino acid carboxy- 0=C — CH2NH2 0=C — CHjNH2 



anhydrides of nucleotides 



(XXXV) 



The schema presented above indicates the formation of a dipeptide, gly- 

 cylglycine. However, if the aimno acids were present along the entire chain, 

 the formation of peptides and consequent Uberation of the amino acids from 

 the polpmcleotide chain might proceed with a single amino acid fixed to a 

 nucleotide. In this way, it is conceivable that the RNA template might 

 possess a number of polypeptides, in various stages of completeness, dangling 

 from the same polynucleotide chain. Such a possibility, in part presented in 

 formula (XXXVI), has been proposed by Dalgleish (1953) and by Borsook 

 (1956), and indeed might be thought of as supported by the work of 

 Koningsberger et al. (1957). 



