Genetic Amino Acid Coding 



439 



little more than one third of these moderate- 

 sized A deletions permit 1589 mutants to 

 show B activity. Consequently, these re- 

 sults strongly suggest that the coding unit 

 is a triplet. 



What can we hypothesize about the na- 

 ture of the spacer that normally interrupts 

 the A and B genes in r+? If the DNA se- 

 quence is interrupted at the ends of each 

 gene, A and B, by short amino acid se- 

 quences (see p. 276), transcription will be 

 physically interrupted, thereby furnishing a 

 starting and a stopping point for the for- 

 mation of messenger RNA and, similarly, 

 polypeptides. Another possibility is the oc- 

 currence in the DNA between the A and 

 B genes of a sequence (or a multiple) of 

 three nucleotides whose complement in mes- 

 senger RNA has no complement in the pre- 

 sumed unique triplet of any sRNA. This 

 untranslatable mRNA codon would make 

 no amino acid sense and is, therefore, called 

 a no sense or nonsense codon. According 

 to this hypothesis, genes A and B normally 

 form one continuous strip of messenger 

 RNA, whose translation produces two sep- 

 arate polypeptides. 



How many of the 64 triplet codons are 

 nonsense? Genetic studies of the rll re- 

 gion strongly suggest that relatively few tri- 

 plets are nonsense. Consequently, most tri- 

 plets probably code for amino acids, and, 

 since only twenty kinds of amino acids com- 

 monly occur, the same amino acid can be 

 coded by more than one codon. Thus, we 

 are apparently dealing in vivo with a degen- 

 erate triplet code. If the base-pairing of 

 sRNA with messenger RNA is strictly ac- 

 curate — that is, exactly complementary — 

 there will be more than twenty kinds of 

 sRNA, several of them carrying the same 

 amino acid. Alternately, if there are only 

 twenty sRNA types, and the base-pairing 

 with messenger RNA triplets is inaccurate, 

 a given sRNA will base-pair with different 

 (but somewhat similar) messenger RNA tri- 



plets. Both of these mechanisms for de- 

 generacy may apply. In any event, most 

 mutants involving base substitutions prob- 

 ably produce sense — that is, code for a dif- 

 ferent amino acid — and therefore produce 

 missense codons. 



It is possible to determine the nucleotide 

 basis for certain point mutants in the rll 

 region.- Suppose that the DNA strand used 

 to make messenger RNA in r+ has a G re- 

 placed by A in a particular r point mutant. 

 If this mutant phage does not lyse the K12 

 strain of E. coli because its messenger RNA, 

 containing a U instead of a C, is abnormal, 

 a defective r+ product results. Although 

 5-fluoro uracil (FU) is not mutagenic when 

 added to the diet of K12, it can be used as 

 a substitute for U when RNA is synthe- 

 sized. When FU substitutes for U in mes- 

 senger RNA, an sRNA molecule may some- 

 times mistake it for C (see discussion of 

 BU on p. 398). If such a mistake is made, 

 the sRNA paired with abnormal messenger 

 RNA will contain G and be the sRNA that 

 transports the amino acid normally found in 

 r+ product. Consequently, the amino acid 

 correct for r+ will be incorporated to form 

 some r+ product, and the host cell will lyse. 

 Therefore, r mutants which can lyse only 

 when FU is added most probably have G 

 on their r+ DNA strand used to make mes- 

 senger RNA, and C on the complementary 

 strand. Those mutants which do not lyse 

 in the presence of FU may have T, A, or 

 C at this locus in the DNA strand used for 

 transcribing messenger RNA. Using vari- 

 ous chemical mutagens as well as FU, it is 

 often possible to determine when T, A, or 

 C is present in the transcribed strand. 



Sometimes a single bacterial mutant si- 

 multaneously suppresses the effects of point 

 mutants at a number of other nucleotide 

 sites. Suppose that in some of these cases, 

 all the suppressed point mutants have the 



- See S. P. Champe and S. Benzer (1962). 



