Genetic Amino Acid Coding 



437 



region may reveal additional information 

 about gene action and the RNA code. 



The A gene has been mapped into six 

 major segments (A, through A 6 ); the B 

 gene into ten (B, through B, (l ), all num- 

 bered consecutively from left to right. Since 

 complementation occurs, a point mutant in 

 any one of the A segments has no effect on 

 B function, and vice versa. A large number 

 of point mutants can be induced in the B, 

 and Bj regions by chemical mutagens ex- 

 pected to cause transitional or transversional 

 base substitutions. In some of these mu- 

 tants all B activity is lost, and in others some 

 product with B activity is detected. As ex- 

 pected, those mutants assumed to involve 

 base substitutions can be reverted to normal 

 B activity by subsequent treatment with 

 chemical mutagens which should produce the 

 reverse transition or transversion. On the 

 other hand, Bx or B 2 mutants produced by 

 acridines always completely inactivate B 

 gene function and are not reverted by mu- 

 tagens assumed to cause base substitutions. 

 Such an occurrence is expected if acridines 

 usually act as mutagens by causing the ad- 

 dition or loss of one or more whole nucleo- 

 tides (see p. 394). 



A large number of B-inactivating, acri- 

 dine-induced mutants located in the B t and 

 B L . segments are obtained. After recombi- 

 nation between such mutants, progeny phage 

 which carry two to six different acridine- 

 induced point mutants are recovered. Some 

 of the doubly-mutant phages still show no 

 B activity, but others do. If a complete 

 series of different double-mutant combina- 

 tions is made, a consistent pattern is ob- 

 served. To interpret it we shall assume 

 that a given single mutant is either + or 

 — , that is, has either gained or lost one or 

 more nucleotides. We shall also assume 

 that a codon has more than two nucleotides 

 and that the code is nonoverlapping; in 

 other words, it is read in successive codons. 

 Operationally, one isolates a " — " mutant 



as a suppressor mutant of a presumed "+" 

 mutant, and vice versa. By isolating a se- 

 ries of "suppressors" and a series of "sup- 

 pressors of suppressors," one gets a series 

 of + and of - mutants. It is not known 

 whether "+" mutations or "— " mutations 

 represent nucleotide additions. Accord- 

 ingly, a double mutant of - — or + + 

 still causes B to have no B activity, since 

 the reading of codons starts to be out of 

 phase with the first mutant and continues 

 out of phase even beyond the second mu- 

 tant. If the mutant loci are widely sepa- 

 rated, we do not expect a double-mutant 

 combination of + - or - ■ + to produce 

 any B activity, since all the codons between 

 the mutants are read incorrectly — out of 

 phase, even though we expect those before 

 the first and after the second mutant to be 

 read correctly — in phase. If, however, a 



H or 1- mutant combination involves 



nearby nucleotides, it is possible that one 

 or only a few codons — those between and 

 including the mutants — will be read incor- 

 rectly. The pattern reveals that any given 

 mutant can be classified either as + or — 

 and that only double-mutant combinations 



of H or (- produce some B activity 



— provided that the two mutants in the 

 Bj-B;. segments are near each other. These 

 assumptions can be tested another way. If 

 a few incorrect codons still permit some B 

 activity, it should be possible to increase 

 the number of mutational errors of the same 

 type (all - or all +) until the number of 

 nucleotides subtracted or added equals the 

 number in a codon. Should this point be 

 reached, the nucleotides beyond the last mu- 

 tated codon would be read correctly — in 

 phase — and some B activity might be re- 

 stored. 



Accordingly, phages carrying three, four, 

 five, and even six different — (or -f) mu- 

 tants are constructed. Some of the three 

 or six multiple - (or + ) mutants have B 

 activity; other combinations, like four — 



