Genetic Amino Acid Coding 



445 



per type of sRNA molecule, the finding '- 

 of approximately forty sites in E. coli DNA 

 which are complementary to sRNA indicates 

 not only the presence of degeneracy at this 

 level but the extent to which it occurs. Al- 

 ready twenty-nine specific sRNAs for six- 

 teen amino acids have been detected in E. 

 coli. li 



Although DNA and the polypeptides it 

 specifies are both linear, it is important to 

 determine whether the exact linearity of the 

 polypeptides is dependent upon the exact 

 linearity of the DNA; that is, whether 

 coli near ity exists. This possibility can be 

 tested using ten <£T4 mutants that produce 

 incomplete head protein molecules. These 

 mutants map in a linear sequence, as deter- 

 mined by recombination studies. When the 

 head protein of each mutant is analyzed, the 

 length of the portion of the molecule made 

 is exactly proportional to the map distance 

 from one end of the gene. This finding 14 

 is proof of colinearity. 



!-By H. M. Goodman and A. Rich (1962), and 

 D. Giacomoni and S. Spiegelman (1962) (see 

 reference on p. 433). 



13 See J. Goldstein, T. P. Bennett, and L. C. Craig 

 (1964). 



14 By A. S. Sarabhai, A. O. W. Stretton. and S. 

 Brenner (1964); see C. Yanofsky, B. C. Carlton. 

 J. R. Guest, D. R. Helinski, and U. Henning 

 (1964), and M. E. Reichmann (1964). 



Despite the degeneracy and ambiguity 

 noted, is the code basically the same for all 

 organisms; that is, is the code essentially uni- 

 versal? It was already mentioned that some- 

 thing very similar to rabbit hemoglobin can 

 be synthesized in a cell-free system derived 

 partly from rabbit reticulocytes and partly 

 from E. coli. As also mentioned, RNA iso- 

 lated from phage f2 directs the synthesis of 

 its coat protein in extracts of E. coli. This 

 RNA also leads to the synthesis of f2 coat 

 protein in extracts of Euglena gracilis. The 

 DNA from the animal viruses polyoma 

 and vaccinia is infective in competent Bacil- 

 lus subtilis; that is, mixing the virus DNA 

 with the bacteria produces intact virus 

 particles which can infect the normal animal 

 host. In certain animal cell-free systems 

 which are stimulated by exogenous RNA 

 messages, synthetic polynucleotides have 

 many of the incorporation properties that 

 they have in bacterial cell-free systems. Fi- 

 nally, a marked correlation exists between 

 C + G content and the percentages of cer- 

 tain amino acids incorporated into protein 

 in a variety of organisms. All these results 

 support the hypothesis that even if there are 

 mutational modifications, only one basic 

 code for polypeptide synthesis exists in all 

 present-day organisms. 



SUMMARY AND CONCLUSIONS 



In vivo study of the rll region of <£T4 reveals that the genetic code for amino acids 

 is read in one direction — probably from one fixed point of messenger RNA— very 

 likely in successive triplets. Such work suggests that the code is degenerate and almost 

 all of the possible codons make sense. 



Studies of polypeptide synthesis in vitro using natural and synthetic messenger RNA, 

 of mutants involving single amino acid substitutions, and of sRNA binding to ribo- 

 somes in vitro support these hypotheses. Such work also permits the assignment of 

 base sequences to the triplets which code in vitro. 



In vivo, DNA and the polypeptide it specifies are colinear; the RNA code is basically 

 universal. 



