Polypeptide Synthesis and RNA 



429 



unpaired bases (such as those making a 

 bobby-pin turn ) in sRNA H-bond with 

 complementary bases in messenger RNA. 

 Transfer RNA would, therefore, also func- 

 tion as an adapter, adapter RNA , leaving 

 the end which carries the transported amino 

 acid — A — C sequence sufficiently flexible to 

 reach a similar region and make a peptide 

 union. 



About 2% of the purine and pyrimidine 

 bases in sRNA are methylated. The methyl 

 groups have their origin in the amino acid 

 methionine (see Figure 32-4, p. 411), 

 which becomes homocysteine upon losing 

 the methyl group. Methylation occurs in 

 bases which are already part of the poly- 

 ribotide of sRNA. Several different en- 

 zymes, called RNA methylases, are involved 

 in the synthesis of 1 -methyl guanine, thy- 

 mine, 5-methyl cytosine, 2-methyl adenine, 

 6-methylaminopurine, and 6-dimethylami- 

 nopurine. RNA methylase is found in the 

 nucleolus and is different in different species. 

 It has been suggested that the methylated 

 bases occupy specific sites in sRNA and 

 somehow play a role in sRNA function. In 

 this connection it should be noted that 

 methylated RNA is more resistant to a 

 potassium-dependent ribo-exonuclease than 

 unmethylated RNA. Some of the bases in 

 ribosomal RNA are methylated. It should 

 be recalled, with respect to DNA, that 5- 

 methyl cytosine is present only in plants and 

 animals, and 6-methyl adenine only in bac- 

 teria. DNA can also be methylated as a 

 polydeoxyribotide, different enzymes being 

 used to form 5-methyl cytosine (from C) 

 and 6-methylaminopurine (from A). 17 



Polypeptide Synthesis 



The 70s ribosome seems to be the smallest 

 unit capable of participating in polypeptide 



synthesis, and observations indicate that the 

 messenger RNA is bound to the 30s subunit. 

 Messenger RNAs of various sizes can asso- 

 ciate with a ribosome. It is likely, however, 

 that only part of a messenger RNA is bound 

 at one time, since ribonuclease in small 

 amounts destroys messenger RNA but not 

 ribosomes. When protein-synthesizing ribo- 

 somes are treated with RNase, the protein 

 being synthesized remains attached to the 

 ribosome. Therefore, the nascent protein 

 is attached to the ribosome, not to messenger 

 RNA. 



Polypeptide chain formation is found 18 to 

 proceed by the step-by-step addition of in- 

 dividual amino acids, beginning at the 

 amino (or N-) terminal end. Consequently, 

 the growing polypeptide chain should end at 

 its carboxyl (or C-) terminal end with an 

 sRNA molecule. Each ribosome has only 

 two sites — both on the 50s subunit — for the 

 attachment of sRNA. When no protein is 

 being made, only one site can hold an sRNA 

 molecule. When protein is being made, 

 however, the second site holds the growing 

 polypeptide chain, which terminates in an 

 sRNA molecule 1 " (Figure 33-3). Conse- 

 quently, each functioning ribosome makes 

 only one polypeptide chain at a time and 

 receives only one messenger RNA at a 

 time. 



How does the ribosome function? Since 

 transfer RNAs apparently form base-pairs 

 with different parts of a messenger RNA 

 strand, it becomes necessary to assume — 

 using the present hypothesis — that each 

 portion of messenger RNA carries its in- 

 formation in unpaired bases at the proper 

 time. The 30s subunit apparently provides 

 an adapter capacity which guarantees that 

 successive segments of messenger RNA are 

 single-stranded and have their bases prop- 



17 See articles by E. Borek; M. Gold and J. Hur- 

 witz; and U. Z. Littauer, K. Muench, P. Berg, W. 

 Gilbert, and P. F. Spahr in Cold Spring Harb. 

 Sympos. Quant. Biol.. 28:139-159, 1963. 



1S From work of H. Dintzes and of R. Schweet 



and collaborators. 



19 See J. R. Warner and A. Rich (1964). 



