Physiological Mechanisms of Resistanxe 171 



p-aminobenzoic acid (PAB) and j9-hydroxybenzoic acid (POB) 

 (Fig. 2). As is well known, PAB utilization is competitively 

 inhibited by analogues in which the carboxyl is replaced by a 

 substituted or unsubstituted sulphonamide group. Similar 

 competition between POB and its sulphonamide analogues 

 was observed. Furthermore, p-nitrobenzoic acid (PNB), an 



•Fig. 2. Schematic representation of competition by analogues 



of PAB and POB. X = 2-thiazolylamino. (Reproduced by 



permission of University of Chicago Press.) 



analogue involving replacement at the other end of the molecule 

 (amino or hydroxyl), competes with both vitamins. It was 

 observed that mutants resistant to PNB competition with 

 either vitamin were not altered in respect to its competition 

 with the other vitamin. Furthermore, with either vitamin 

 there was no crossresistance between PNB competition and 

 sulphonamide competition. Without presenting in detail the 

 arguments involved or the subsidiary evidence (cf. Davis and 

 Maas, 1952), I should like to summarize by stating that this 



