430 



CHAPTER 47 



nucleus contains an enzyme, RNA poly- 

 merase, which is necessary for RNA syn- 

 thesis. This RNA synthesis can be per- 

 formed in vitro, and requires also the pres- 

 ence of DNA, as well as all four riboside 

 triphosphates; the RNA synthesized proves 

 to have the same base ratio as the DNA 

 (except that T is U). This situation is remi- 

 niscent of the synthesis of DNA, where the 

 DNA polymerase takes directions from 

 single-stranded DNA. Here RNA poly- 

 merase takes directions from DNA in the 

 making of RNA polymer. It has been found 

 that single-stranded DNA can serve as tem- 

 plate for the RNA polymerase, and that an 

 RNA polymer probably cannot serve as 

 primer. 



5. How do the amino acids destined to 

 form polypeptides arrive at the ribosomes? 

 Whereas ribosomal RNA has a relatively 

 high molecular weight (Jo to 1 million), as 

 already mentioned, there is another kind of 

 RNA in the cytoplasm which has the rela- 

 tively low molecular weight of about 18,000 

 (having about 30 bases). Other evidence 

 indicates that there are 80-90 bases per 

 molecule. Since this RNA is soluble when 

 ribosomal RNA is not, it is called soluble 

 RNA or sRNA. There is also no experi- 

 mental proof that sRNA is derived from 

 nuclear RNA. Using radioactively labeled 

 amino acids it can be demonstrated that the 

 amino acids arrive at the ribosomes singly, 

 each attached to a molecule of soluble RNA. 

 All the soluble RNA molecules are similar in 

 their terminal nucleotides, one end terminat- 

 ing with the base G and the other end with 

 the base sequence — C — C — A. Otherwise, 

 they are dissimilar, being of about 20 dif- 

 ferent types, each capable of carrying a differ- 

 ent one of the amino acids typically found in 

 protein. The transported amino acid is at- 

 tached through its carboxyl ( — COOH) 

 group to the 2' or 3' hydroxyl group of the 

 terminal adenosine. 



Since sRNA serves to transfer the amino 



acids to the ribosomes, it can also be called 

 transfer RNA. 



6. What do we know about the formation 

 of the amino acid-transfer RNA complex? 

 It has been found that each of the 20 amino 

 acids must be activated before it can be ac- 

 cepted by its particular transfer RNA. Both 

 the activation of an amino acid and its at- 

 tachment to soluble RNA may involve the 

 activity of a single enzyme. There is prob- 

 ably a different activating enzyme for each 

 kind of amino acid. Activation involves 

 the combination of the amino acid at its 

 carboxyl end to the adenosine triphosphate 

 (ATP) located terminally, with the removal 

 of two phosphates as pyrophosphate. This 

 reaction can be summarized: amino acid 

 + ATP ^ amino acid adenylate + pyro- 

 phosphate. 



7. What determines which transfer RNA 

 molecules are to be attracted to a ribosome? 

 It has been mentioned already that, after a 

 phage infects its host, RNA is made having 

 the base ratio of the phage DNA. It has 

 been found that polynucleotides of this phage- 

 specific RNA become attached to a small 

 percentage of already formed ribosomes. 

 This suggests that many ribosomes do not 

 automatically carry a template of RNA, 

 containing information for specifying amino 

 acid sequence, which it obtained from the 

 DNA template. Such ribosomes are capable 

 of receiving segments of template RNA 

 which carry the code for making phage- 

 specific polypeptides. Thus, there is a third 

 type of cytoplasmic RNA, messenger RNA, 

 which carries information for cistronic action 

 from phage DNA to the ribosome. It is the 

 messenger RNA which attracts the various 

 molecules of transfer RNA, each of which is 

 carrying individual amino acids. It has also 

 been shown ^ that messenger RNA is used 

 in sending cistronic template information 

 from the regular DNA content of a cell to its 



«" By M. Hayashi and S. Spiegelman (1961). 



