18 



P-AMINOBENZOIC ACID 



tion of phenylalanine, tyrosine, tryptophan, and PABA. For rapid growth 

 one such strain recjuires, in addition, p-hydroxybenzoic acid (POB).'" 

 Certain other mutants, blocked further along in the pathway of aromatic 

 synthesis so that they require tyrosine and phenylalanine for growth, ac- 

 cumulate shikimic acid and another compound (compound X) in the me- 

 dium.^' '"■" 



Compound X has been isolated in crystalline form and is characterized 

 as follows: m.p. 150 to 152°; [af^ - 52.1 d= 1.0°; X„,ax (in ethanol) 233 m/x; 



3CH3COOH 



pantothenic acid 

 (coenzyme A?) 



HO 



OH 



Unknown 

 intermediates 



OH 



Phenylalanine 

 Tryptophan 

 Tyrosine 

 PABA 



log, 4.0; pK 3.2; C, 48.83; H, 4.85; molecular weight 162; neutralization 

 equivalent 162, 179; reduces Fehling's and Tollen's reagents; forms crystal- 

 line semicarbazone, phenylosazone, methyl ester, and acetate. Compound 

 X has recently been identified as a 5-dehydroshikimic acid." 



A tentative and necessarily incomplete outline of aromatic biosynthesis 

 in Escherichia coli may be summarized according to Davis as follows: 



COOH 



COOH 



Unknown 

 precursor (s) 



Aromatic 

 amino acids 



/ 



0" ^ OH 



OH 

 Compound X 

 (5-dehydro- 

 shikimic acid) 



Evidence has been presented by Uavis^^ indicating that PABA partici- 

 pates in the synthesis of vitamin B12 , since PABA has a sparing effect on 

 the B12 requirement of certain Escherichia coli mutants. Although a cata- 



10 B. D. Davis, Natur.e 166, 1120 (1950). 



" I. I. Salamon and B. D. Davis, Proc. 119th Meeting, Am. Chem. Soc. Boston (1951). 



" B. D. Davis, ./. Bacteriol. 62, 221 (1951). 



