IV. BIOCHEMICAL SYSTEMS 135 



and S. durans), one member of which required PGA and the other did not, 

 were compared for their sulfonamide sensitivity by Lamj^en and Jones.'* 

 None of the cuhures requiring preformed molecule were inhibited l)y 1280 

 7 of sulfadiazine per milliliter. Those members of strains which were able to 

 synthesize their own POA were inhi])ited by 0.3 jug. of sulfonamide per 

 milliliter but became insensitive when supplied with an amount of pteroyl- 

 triglutamic acid equal to that used to promote growth of PGA-requiring 

 members of the same strain. 



Thus in the case of the lactic acid bacteria and the Streptococci the evi- 

 dence is in harmony with the thesis that p-aminobenzoic functions solely 

 in the synthesis of PGA and that sulfonamide inhibits this conversion. 



6. PGA behaves as a non-competitive sulfonamide antagonist for CI. 

 acetobutylicum (two strains) and Streptohacterium plantarum (three strains). 

 These organisms, which require p-aminobenzoic acid, will also respond to 

 ten to one hundred times as much PGA. The amount of PGA necessary to 

 reverse the sulfonamide is approximately the same as that required to pro- 

 mote growth in its absence and is independent of the amount of sulfonam- 

 ide.^^ This is similar to results obtained with Streptohacterium plantar um.^^ 



7. The Le. citrovorum factor (fohnic acid) is able to replace p-amino- 

 benzoic acid for growth of L. mesenteroides and overcomes sulfonamide in- 

 hibition in a non-competitive manner. Neither PGA nor N^^-formyl PGA 

 is active in this respect. These observations of Lascelles et aU'' lend support 

 to the view that the inactivity of PGA in replacing p-aminobenzoic acid 

 may reflect the inability of the organism to convert preformed PGA to the 

 more metabolically active form. 



b. Biochemical Systems Where PGA Cannot Replace p-Aminobenzoic Acid 



1. A p-aminobenzoic acid-less mutant strain of E. coli was observed 

 which would not respond to PGA.^^ Growth could be induced either by low 

 concentrations of p-aminobenzoic acid (0.2 7 per milliliter) or by a combina- 

 tion of amino acids, purine, and thymine. This mutant strain, when grown 

 in the presence of 0.01 7 of p-aminobenzoic acid per milliUter, produces a 

 material giving a growth response with S. faecalis on a PGA-free medium. 



With other organisms, such as Acetobacter suboxydans 621 and a Neuro- 

 spora mutant, which require p-aminobenzoic acid, PGA reverses the action 

 of sulfonamide in a non-competitive mainier.^^ 



2. PGA is unal)le to affect the inhibition ratio of sulfanilamide for E. 

 coli at the levels where methionine, purine, or serine synthesis is affected. 

 At the level of thymine synthesis, PGA is effective.^- '" The lack of activity 



^« K. II. Ximino-Siiiilh and 1). I). Woods, J. Gen. Miciobiul. 2, .x (1948). 



" J. Lascelles, M. J. Cross, iind I). D. Woods, Biochein. J. 49, Ixvi (1951). 



5« J. O. Lampen, M. J. Jones, and R. R. Roepke, /. Biol. Chem. 180, 423 (1949). 



