438 



CHAPTER 34 



and one I (or four -)- and one ) mu- 

 tants also show B activity. None is found 

 if the mutants fail to add up to three or a 

 multiple of three. These results demon- 

 strate that the message from gene B is trans- 

 lated via successive, nonoverlapping codons 

 and that a codon is most probably three 

 successive nucleotides. The proposed mo- 

 lecular model of sRNA whose turn is pre- 

 sumably made by three unpaired nucleo- 

 tides (see Figure 33-2, p. 428) is consistent 

 with this statement. Apparently, the triplet 

 codon of DNA is transcribed into a com- 

 plementary triplet codon of messenger RNA, 

 which, in turn, is translated into an amino 

 acid brought into position by an sRNA mol- 

 ecule bearing a complementary triplet co- 

 don. The triplet codon in DNA and a 

 unique triplet in sRNA are therefore ex- 

 pected to be identical, except that T in the 

 former is U in the latter. 



In r + , genes A and B are separated by 

 a spacer which results either in separate 

 messenger RNAs for each gene or a non- 

 meaningful segment between the A and B 

 parts of a single messenger RNA. One par- 

 ticular deletion, number 1589, removes most 

 of region A.-, and all of regions A, ; , B u and 

 B 2 . Such phage particles show no A but 

 partial B activity. Whether or not r + 

 makes separate messenger RNAs for the A 

 and B genes, the spacer denoting the end 

 of the A message and the start of the B 

 message (or the reverse, the end of B mes- 

 sage and the start of A message) may be 

 absent in phages carrying deletion 1589. 

 Consequently, this mutant may make only 

 one continuous strip of messenger RNA 

 containing the base complements of those 

 parts of the A and B genes still present. 

 This possibility can be tested as follows: 

 single + (or — ) acridine-induced mutants 

 in the A region are introduced by recom- 

 bination into phages carrying deletion 1589. 

 In each case the B gene is rendered inac- 

 tive. In other words, B gene activity is 



now vulnerable to mutants in the A gene. 

 This finding supports the view that dele- 

 tion 1589 enables two genes to form one 

 messenger RNA (whether or not they do 

 so in r + ) and that if the reading is out of 

 phase due to a nucleotide addition or sub- 

 traction in A. all subsequent codons — that 

 is, those in gene B — will be misread. This 

 result also suggests that the codons in ill 

 messenger RNA are always read from A 

 toward B, the order in which the genes are 

 usually represented in genetic maps (as in 

 Figure 26-4, on p. 343). Other evidence 

 supports this interpretation. For example, 

 when we make deletion 1589 phage doubly- 

 mutant in the A region, the B gene is in- 

 activated in some cases, but in others some 

 B activity is detected. When + and — are 

 assigned the mutants, we find that only -f- - 

 double mutants in A can restore B activity; 

 — — or -| — \- combinations cannot. More- 

 over, as expected, the sequence -\ or 



- + and the distance between these two 

 mutants make no difference. 



Since deletion 1589 has some B activity, 

 it must be associated with the deletion of 

 some multiple of three nucleotides. Al- 

 though the codon cannot be less than three 

 nucleotides, it can be a multiple of three 

 if, for example, each + (or — ) mutation 

 added (or lost) two nucleotides. In this 

 case the codon will be six nucleotides. We 

 can test for the size of the codon by com- 

 bining mutant 1589 with different medium- 

 sized deletions in the A gene. Assuming 

 that the two breakage points involved in 

 such deletions occur at random, then only 

 one third of the A deletions should remove 

 exact multiples of three nucleotides, only 

 one sixth should remove exact multiples of 

 six, and so on. Therefore, a test of these 

 deletions should show one sixth which per- 

 mit the B gene to function if the codon is 

 six nucleotides long; one third should per- 

 mit B to function if the codon is three nu- 

 cleotides long. Tests show, in fact, that a 



