SELECTED PAPERS 



ascospores of an ascus are of two kinds, four like one parent and four 

 like the other. It is evident that this situation provides a means to 

 check whether a property characteristic for some induced mutant is 

 dependent on the mutation of a single gene. 



Beadle and his collaborators have now succeeded in obtaining a 

 large number of biochemical mutants by treating asexual spores with 

 X-rays or ultraviolet light, making crosses with strains of the opposite 

 sex, and then establishing genetically homogeneous, single ascospore 

 strains. Special attention has now been given to the question whether 

 these strains differed in their growth requirements from the original, 

 wild-type strain. For the latter it had previously been established that 

 it grew quite satisfactorily in a medium containing some sugar, the 

 necessary inorganic salts - nitrate as a source of nitrogen - and biotin 

 as the only indispensable growth factor. For many of the mutants ob- 

 tained it was found that they no longer developed in this minimal 

 medium, but that they did so perfectly well if some extract containing 

 the various amino acids and B-vitamins was added. This result can 

 only be understood on the basis of the assumption that the ability of 

 the wild-type strain to synthesize one or more of the essential amino 

 acids and B-vitamins was lost in the mutation act. By composing a 

 series of media, in each of which a single amino acid or one of the 

 growth factors had been added to the minimal medium, and observing 

 in which case growth occurred after inoculation with the mutant 

 strain, it was, of course, possible to establish for this particular strain 

 the specific loss in synthetic ability. By this procedure it could be 

 shown that amongst the numerous mutants obtained there were 

 strains which differed from the wild-type strain by loss of the ability 

 to synthesize one of the following compounds : thiamine, pyridoxine, 

 p-aminobenzoic acid, pantothenic acid, inositol, nicotinic acid, cho- 

 line, arginine, lysine, leucine, valine, methionine, tryptophan, proline 

 and threonine. Moreover, the genetical analysis showed that each of 

 these strains differed from the wild-type by a single gene, the normal 

 allele of which is evidently essential to the biosynthesis of the com- 

 pound in question. 



A further biochemical investigation of some of these mutant strains 

 has led to some extremely interesting results. It is clear that the bio- 

 synthesis of each of the compounds mentioned depends on a long chain 

 of individual step-reactions, and that the blocking of any of these step- 



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