»2S 



( IIAPTER 33 



Paired Bases — / 



Unpaired Bases 

 in Turn 



Unpaired Bases 

 at Terminus 



FIGURE 33-2. 

 Proposed structure 

 for adapter RNA. 



and its sides contain about thirty base-pairs. 

 It is calculated that no less than three un- 

 paired nucleotides are needed to make the 

 turn. Each of these RNA molecules is also 

 similar, being about 100A long and 20A 

 wide. In other respects, they are dissimilar: 

 they are believed to differ internally in base 

 content or sequence, and comprise about 

 twenty different types, each capable of carry- 

 ing a different one of the amino acids typically 

 found in protein. Since sRNA forms base 

 pairs best with denatured homologous DNA, 

 the sRNAs of different species probably vary 

 in their nucleotide sequences. 1 ' Because 

 sRNA acts to transport the amino acids to 

 the ribosomes, it is also called transfer RNA. 

 It is found that to form the amino acid- 

 transfer RNA complex, each of the twenty 

 acids must be activated before it is attached 



ir, See D. Giacomoni and S. Spiegelman (1962), 

 and S. Spiegelman (1964). 



to its particular transfer RNA. Activation 

 involves the combination of the amino acid 

 at its carboxyl end to the 2' or 3' hydroxyl 

 group of adenosine triphosphate (ATP) 

 with the removal of two phosphates as pyro- 

 phosphate. This reaction can be summa- 

 rized as: amino acid + ATP ?^ amino acid 

 adenylate + pyrophosphate. The attach- 

 ment reaction can be summarized as: 

 sRNA + amino acid adenylate %± amino 

 acyl sRNA -f- adenylic acid. Both the ac- 

 tivation of an amino acid and its attachment 

 to soluble RNA may involve the activity of 

 a single enzyme, probably a different one 

 for each kind of amino acid. 1 " 



Each amino acid to be incorporated into 

 a polypeptide is attached to a specific sRNA 

 when it arrives at the ribosome. Since sRNA 

 is probably not completely base-paired with 

 itself, we can suppose that three (or more) 



" ; See A. T. Norris and P. Berg (1964). 



