SYNTHESIS OF AMINO-ACIDS 67 



and agar containing known additional nutrients is layered 

 over the original plate. Any new colonies which develop 

 after further incubation are derived from mutant spores, and 

 after being subcultured their nutrition can be studied in 

 more detail. Special techniques may be required in order 

 to promote the formation of discrete colonies, particularly 

 with organisms whose growth tends to spread (many strains 

 of Neurospora). A more laborious method of isolation is to 

 subculture each ascospore on a rich medium, i.e. one con- 

 taining amino-acids and gro-wth factors, and then transfer 

 to a minimal medium. If there is no growth on the latter, 

 the nutrition of the parent colony is then examined further. 

 Other more efficient, technically easy and less laborious 

 methods for selecting mutants of various organisms have 

 been described [18, 31, 32]. 



Using such techniques, a number of mutants have been 

 obtained which are exacting towards a particular amino- 

 acid, growth factor, purine or pyrimidine. Many appear to 

 be unable to perform reactions expected to take place in 

 one step, e.g. the amination of inosine to form adenine, and 

 Beadle and Tatum have advanced the hypothesis that each 

 enzyme is controlled by a specific gene, and any change 

 in the latter is reflected by an alteration in the enzyme's 

 activity. Although the evidence is indirect and has been 

 criticized by Delbruck [11] an analysis of the available infor- 

 mation shows that at least 73% of the genes of Neurospora 

 have only one function, and there are no indisputable 

 examples of genes with two or more functions [30]. It must 

 be stressed that mutation may involve modification rather 

 than complete loss of the gene and the corresponding 

 enzyme, e.g. a Neurospora mutant unable to synthesize 

 tryptophan from indole and serine still possessed the requi- 

 site condensing enzyme, though in an inactive state [22]. 

 (Cf. also the synthesis of pantothenic acid [51]). 



In order to show whether mutants requiring the same 

 factor are due to mutation of either the same gene or com- 

 pletely different genes, one of two tests may be applied. In 

 organisms with a sexual cycle (e.g. Neurospora), each mutant 

 is mated with a parent of known genetic composition and the 



