43^ PLANT GROWTH SUBSTANCES 



years that certain drugs act in this fashion. The sulfonamides, for 

 example, competitively inhibit transformation of /^-aminobenzoic acid 

 to one or more essential catalysts in those bacteria which they inhibit. 

 Their effects can be overcome either by addition of /7-aminobenzoic 

 acid, which acts competitively to overcome the effects of the drug, or by 

 a variety of other substances (folic acid, thymine, purine bases, serine, 

 methionine), which act noncompetitively (26,44). Each of these sub- 

 stances might be considered a new growth factor required by the cells 

 under conditions where a single essential reaction has been inhibited, 

 in this case by an inhibitor (a sulfonamide) foreign to the cell. 



This concept of the origin of some nutritional requirements presents 

 several interesting possibihties. Examples of "nutritional symbiosis" 

 (possibly related to true symbiosis), in which each symbiont supplies by 

 synthesis an essential nutrient for the other symbiont, are easily observed 

 experimentally (29). Perhaps a new type of nutritional symbiosis, in 

 which one symbiont metabolizes to its own advantage a product elabo- 

 rated by the second symbiont and which would otherwise inhibit growth 

 of the latter by preventing essential synthetic reactions, may be observed. 

 Perhaps the toxicity of some types of organic matter for certain auto- 

 trophs may be exerted in this fashion. This point of view should also 

 make possible a different approach to the discovery of new growth 

 factors. For if many nutritional requirements arise naturally through 

 the inhibition of essential synthetic reactions, then one might purposely 

 inhibit growth of bacteria with all types of organic compounds. Wherever 

 these toxic effects could be reversed by addition of natural materials, 

 the possibility would present itself that synthesis of some metabohcally 

 essential compound, supplied by the natural material, was the process 

 limiting growth in the presence of the inhibitor. Isolation of the com- 

 pound or compounds effective in the reversal might then bring to light 

 new compounds of importance in metabolism. It was in this way that 

 /?-aminobenzoic acid was isolated; a naturally occurring substance which 

 overcame the toxic action of sulfanilamide was observed (45), and on 

 isolation, proved to be /^-aminobenzoic acid (27). More recently Shive 

 and coworkers (30) isolated thymidine by this approach. This compound 

 proved to be one of the naturally occurring substances which prevented 

 the toxic action of 7-methylfolic acid for Escherichia coli. The approach 

 has never been systematically exploited, however. 



We have seen that growth of Saccharomyces carUbergensis 4228 is 



