15. II. 1050] 



Deficient Bacterial Mutants Isolated 



least in the presence of penicillin are sufficient to 

 limit the permissible population density in the penicillin 

 method of mutant isolation ; this procedure is appar- 

 ently much more sensitive to traces of syntrophism 

 than is the technique of adjacent streaks. 



(3) Conversion of a precursor. It has been observed 

 that wild-type, growing on an excess of the keto acid 

 precursor of isoleucine, will feed a mutant that responds 

 to isoleucine but not to its keto acid. The same con- 

 version and excretion is carried out by a mutant, 

 blocked earlier, which can use this precursor as well as 

 isoleucine itself. It has also been observed with wild- 

 type acting on certain other precursors. In some 

 reactions, however, such as the conversion of ornithine 

 or citrulline to arginine, it has not been possible with 

 an excess of either precursor to stimulate the excretion 

 of arginine by wild-tvpe or by a mutant. It is evident 

 that the organism possesses more than one type of 

 mechanism for determining the rate of synthesis of an 

 amino acid ; in the case of isoleucine, the capacity for 

 amination of the keto acid exceeds the requirement, 

 and hence cannot be the rate-governing mechanism. 



A +- »■ specific thiazole . 

 B -\— ► specific pyrimidine ' 



C -] — ► pantonine -\— >• pantoic acid, 

 D -|— »• /S-alanine 



pantothenic acid 



Fig. 3. - Synthesis of thiamine and pantothenic acid. Blocked arrows 

 represent sites of genetic blocks. 



(4) Excretion of a lone conjugant. Thiamine is 

 composed of two moieties, a substituted thiazole and a 

 substituted pyrimidine (Fig. 3). Thiamine-less mutants 

 were obtained which respond respectively to the 

 pyrimidine, to the thiazole, and to neither (implying a 

 deficiency in the conjugation of the two) ; a fourth type, 

 whose site of genetic deficiency is not readily inter- 

 preted, responds to the specific thiazole plus pyrimi- 

 dine, as well as to thiamine itself, but not to either 

 moiety alone. Tests for syntrophism showed that the 

 thiazole-less mutant feeds the pyrimidine-less mutant, 

 but not vice versa. Mutants with similar blocks and 

 accumulations have been observed with Neurospora 1 . 



Another vitamin, pantothenic acid, also consists of 

 two components, pantoic acid and /J-alanine (Fig. 3). 

 W. K. Maas in this laboratory has isolated various 

 pantothenic acid-less mutants which respond respect- 

 ively to pantoic acid or pantonine; to /S-alanine; to 

 pantothenic acid only; and to a mixture of pantoic 

 acid and /5-alanine. The pantoic-less mutant feeds the 

 /3-alanine-less mutant, but not vice versa. 



In both these instances of syntrophism, the cause of 

 the accumulation of the intermediate is not absence of 



the enzyme concerned with its further conversion, but 

 inability of that enzyme to perform the conversion 

 in the absence of the second conjueant. 



(5) Release of physiological brake on a synthetic 

 process. One of the most interesting observations to 

 turn up is mutual syntrophism between certain 

 tyrosine-less and phenylalanine-less mutants. Paper 

 chromatography confirmed the inference that the 

 block in phenylalanine synthesis resulted in excretion 

 of tyrosine itself, rather than a precursor, while a 

 block in tyrosine synthesis caused excretion of phenyl- 

 alanine. Feeding of a tyrosine-less mutant by culture 

 filtrates of a phenylalanine-less mutant has also been 

 observed with another strain of E. coli 1 . 



From the presence of mutants with a double re- 

 quirement for these two compounds, as well as others 

 with triple and quadruple requirements for these plus 

 other aromatic compounds, all resulting from single 

 mutations, it had been inferred that phenylalanine 

 and tyrosine arose from a common precursor (Fig. 4). 

 A possible explanation for the mutual syntrophism 

 therefore appeared to be the diversion of this precursor 

 in one rather than the normal two directions, with 

 resultant excretion of the excess of phenylalanine or 

 tyrosine. Further study, however, showed this explan- 

 ation to be inadequate, as the amount of phenylalanine 

 (or tyrosine) excreted was much larger than the amount 

 of tyrosine (or phenylalanine) consumed. Since the 

 requirements of mutants of E. coli for these two 

 compounds are of the same order of magnitude, simple 

 diversion could not account for the large production. 



An alternative explanation has been developed, 

 stimulated largely by the interesting work of Bonner 2 . 

 He showed that a single block in isoleucine synthesis, 

 between the a-keto and the amino acid, accounted for 

 the double requirement of a Neurospora mutant for 

 isoleucine and valine. Apparently the accumulated 

 isoleucine precursor competes as a structural analogue 

 with the corresponding compound in the valine-chain, 

 causing a requirement for this amino acid as well: 



\ — ► ketoisoleucine »- isoleucine 



Recently Adelberg 3 has found that the compound 

 accumulated by this mutant is not ketoisoleucine, but 

 is rather the a-/?-dihydroxy acid. The principle of 

 internal inhibition by a normal metabolite, however 

 remains unchanged. 



This evidence of inhibition of a normal reaction by 

 increased concentration of a normal metabolite has 

 seemed to us to point to the possibility of a general 

 mechanism of integration of various parallel sequences 



Biol. CI 

 g, perso 



