Polypeptide Synthesis and RNA 



427 



megaterium furnish evidence " that different 

 segments of DNA are complements of 16s 

 and 23s ribosomal RNA. Although com- 

 pletely-formed ribosomes do not accept large 

 quantities of newly-formed RNA. labeling 

 experiments show that a small amount of 

 RNA turnover is associated with complete 

 ribosomes. This turnover RNA is mRNA. 

 Accordingly, the greater part of the RNA in 

 ribosomes — ribosomal RNA — is usually in- 

 corporated at the time of ribosome forma- 

 tion. The precise mechanism by which 

 RNA is incorporated into new ribosomes as 

 ribonucleoprotein is still unknown. 



The wild type of the toad Xenopus laevis 

 usually has two nucleoli in its diploid cells. 

 A recessive lethal mutant, called anucleolate, 

 has one nucleolus when heterozygous and 

 none when homozygous in which case the 

 mutant has many small, nucleolar "blobs" 

 instead of typical nucleoli. The mutation 

 apparently involves the nucleolus organ- 

 izer 12 — the region of the chromosome re- 

 sponsible for nucleolus formation (p. 11). 

 The homozygous mutant also fails to syn- 

 thesize ribosomal RNA which, in this or- 

 ganism, includes 18s and 28s types although 

 small RNA molecules (4s), probably other 

 RNA, and DNA continue to be made. 

 Homozygous mutants, just as the normal 

 homozygotes, conserve and apparently utilize 

 the ribosomes contributed in the egg. The 

 heterozygote produces the same amount of 

 28s and 18s ribosomal RNA as the normal 

 homozygote. Since these two kinds of ribo- 

 somal RNA differ in base composition, they 

 are probably products of separate DNA se- 

 quences. The synthesis of both types, how- 

 ever, is prevented by the single mutant whose 

 molecular basis is unknown. 



11 See S. A. Yankovsky and S. Spiegelman (1963), 

 and S. Spiegelman (1964). 



12 The subsequent discussion follows D. D. Brown 

 and J. B. Gurdon (1964). and E. H. McConkey 

 and J. W. Hopkins (1964). 



Since ribosomal RNA is complementary 

 to about 0.6% of deoxyribotide pairs in bac- 

 teria and the mouse, many genes apparently 

 are involved in the synthesis of ribosomal 

 RNA. The results with anucleolates sug- 

 gest that these genes are linked closely and 

 can be controlled by a single genetic change. 

 The DNA attached to nucleoli isolated from 

 Hela human-tissue culture cells can be col- 

 lected and heat denatured. Such single- 

 stranded DNA forms molecular hybrids with 

 ribosomal RNA. This result (together with 

 others) shows that a concentration of DNA 

 complementary to ribosomal RNA occurs in 

 the nucleolus organizer region. 



Transfer (Soluble or Adapter) RNA 



As already mentioned, ribosomal RNA has 

 a relatively high molecular weight (about 

 one-half to one million). The cytoplasm 

 contains another kind of RNA which has 

 the relatively low molecular weight of about 

 18,000 and consists of about 67 nucleotides. 

 Since this RNA is soluble in ~ 1M NaCl 

 and ribosomal RNA is not, it is called soluble 

 RNA or sRNA. sRNA is probably also de- 

 rived from nuclear RNA. 13 The use of 

 radioactively-labeled amino acids shows that 

 the amino acids arrive at the ribosomes in- 

 dividually, each attached to a molecule of 

 soluble RNA. All the soluble RNA mole- 

 cules are similar in their terminal nucleo- 

 tides, one end terminating with the base G 

 and the other end with the base sequence 

 — C — C — A, and X-ray diffraction studies 

 show them to be primarily in double-helix 

 condition. 14 Since each sRNA is composed 

 of a single strand, the molecule must be in 

 the shape of a twisted "bobby pin" (Figure 

 33-2 and Suppl. IV, Fig. 12) or some sim- 

 ilar highly base-paired configuration. The 

 bobby pin terminates unevenly at the ends. 



13 See M. I. H. Chipchase and M. L. Birnstiel 

 (1963). 



14 See G. Zubay (1963). and Suppl. IV. 



