43^ PLANT GROWTH SUBSTANCES 



(or hypoxanthine) in the medium which contains guanine apparently 

 results not from absence of the appropriate enzyme but from inhibition 

 of some step in the synthesis by guanine. Beerstecher and Shive (4) 

 found, similarly, that tyrosine inhibited growth of a strain oi Escherichia 

 coli under appropriate conditions and that this inhibition was alleviated 

 by phenylalanine. Apparently tyrosine inhibited synthesis of phenyla- 

 lanine by this organism, a synthesis which occurred without difficulty 

 when tyrosine was omitted from the medium. 



A particularly clear-cut example of a nutritional requirement which 

 results from an inhibition rather than from lack of the enzymes necessary 

 to carry out a synthetic process is provided by the yeast, Saccharomyces 

 carlsbergensis. Under ordinary cultural conditions this yeast requires 

 vitamin Be for growth, and its growth response to additions of this vita- 

 min has been the basis for a widely used method for determining vitamin 

 Be- Yet if thiamin is omitted from the culture medium and the inoculum 

 has been grown in a thiamin-low medium, the organism grows well in 

 the complete absence of vitamin Be (Fig. 2), and analyses of the cells 

 have shown that vitamin Be is synthesized under these conditions (25). 

 Addition of rather small amounts of thiamin — a substance which is itself 

 required in metabolism and is normally present in this yeast — inhibits 

 growth, and under these conditions vitamin Be is required to permit 

 growth (Fig. 2). How thiamin inhibits growth and how this inhibition 

 is counteracted by vitamin Be are both unknown. The important point 

 is, however, that a growth factor which under one set of conditions is 

 synthesized in amounts sufficient for growth cannot be thus synthesized 

 under another slightly different set of conditions, or is synthesized in 

 amounts insufficient for growth. Thus the nutritional requirement for 

 the growth factor results from an inhibition rather than from lack of the 

 enzymes to accomplish a given synthesis. 



From these examples it is evident that the failure of an organism to 

 synthesize an essential growth factor may result either from lack of the 

 necessary enzymes to effect the synthesis, or from inhibition of one or 

 more synthetic processes by metabolites normally present within the 

 cell, or added with the medium. In general terms, if b is essential for 

 synthesis of protoplasm and is normally formed from certain precursors, 

 a, by an enzymatic process, then the synthesis: 



a > b 



may be eliminated either by lack of one of the necessary enzymes or by 



