96 growth: synthesis of bacterial protoplasm 



Escherichia coli, but tlie early work which involved genetical 

 analysis of the mutants produced was carried out with 

 Neurospora. The synthesis of cellular material occurs as the 

 end-result of a chain of reactions, each catalysed by a specific 

 enzyme. If an essential cell constituent D is synthesised from 

 the raw food-stuff A by a series of steps A^B-^C-^D, then 

 the cell will be able to grow if it is supplied with any of the sub- 

 stances A, B, C, or D. If, however, the enzyme catalysing the 

 formation of C from B is inactivated, then the organism will be 

 able to grow provided it is supplied with either D or C but not 

 if supplied with A or B. If we can find three mutants of 

 the organism, each of which has lost one of the enzymes 

 involved in the synthesis of D from A, then, by studying the 

 nature of substances necessary for growth of the mutants, 

 it should be possible to reconstruct the chain A->B->C->D. 

 This is the principle of the method of " biochemical 

 mutants." 



Irradiation of Neurospora results in the production of 

 mutants; the absorption of one quantum of radiant energy 

 causes an alteration of one gene which causes the loss, in turn, 

 of one enzyme. Some of the mutants produced will have lost 

 an enzyme involved in the synthesis of an amino-acid X. 

 All the " wild type" organisms can grow on a medium which 

 contains ammonium ions as N-source but the mutants will 

 only grow if the amino-acid X is added to the medium. 

 Consequently the organisms obtained after irradiation are 

 examined for their ability to grow (1) on a medium containing 

 ammonia as sole N-source, and (2) on a medium containing 

 ammonia -|- X as N-source. All those organisms which grow 

 on (1) can be discarded, while those which grow only on (2) 

 are further investigated. In the first place the number of 

 genetic types present is determined ; this gives an idea of the 

 number of genes, and consequently enzymes, involved in the 

 synthesis of X. Next the possible precursors of X are tested 

 as nutrients in place of X. 



For example: seven different mutants of Neurospora were 

 isolated by Srb and Horowitz and were found to require 



