318 MUTATION AND PLANT BREEDING 



strains, high yields or no yield (85, 90). Complex patterns of resistance 

 to virulent phage, production of temperate phage, and growth 

 inhibition by bacteriocins occur among actinomycetes (17, 23, 98, 

 112). The occurrence of lysogeny raises the possibility of transferring 

 characteristics between strains by transduction or conversion with 

 temperate phage (7, 101). Survivors of a phage exposed population 

 have a varied cobalamin vield (65). The usefulness of transduction is 

 unexplored in strain selection. The success of such a program would 

 depend upon the efficiency of the screening technique used to detect 

 change. As an example, consider a series of strains resistant to high 

 levels of an antibiotic that is produced by most of the strains at low 

 levels, and at a high level by at least one strain. Production by the 

 low level strains may be inhibited by genetic factors replaceable with 

 different alleles from the production level strain. With a different 

 genetic constellation, these strains may produce antibiotic at yields 

 above the level of the former production strain. 



Various correlations between physiological properties during 

 fermentation and yield have been noticed but are usually specific to 

 the strain and conditions used and cannot be generalized or adapted 

 for use as a screening method. Correlations between catalase activity 

 and streptomycin production (72) and fluorescence and tetracycline 

 production (88) have been described. 



Nutritionally exacting variants, auxotrophs, of antibiotic- 

 producing microorganisms can be obtained by modifications of 

 screening methods applied to Neurospora and Escherichia. Auxo- 

 trophs described in the literature have been obtained for use as selec- 

 tive and differential genetic markers in recombination studies, and 

 groups of mutants blocked in the production of known metabolites 

 have been used to determine pathways of biosynthesis. Two differ- 

 ences exist between these studies and possible applications to anti- 

 biotic production. Biosynthetic pathways blocked in auxotrophs lead 

 to metabolites that are completely utilized during growth for further 

 synthesis of more complex substances, there being little or no over- 

 production and excretion of the metabolite into the medium by non- 

 mutants, as occurs with antibiotics, and none of these metabolites 

 serve as necessary intermediates in the biosynthesis of antibiotics of 

 major commercial importance with the exception of penicillin. Thus, 

 the selection of auxotrophs of a given biochemical class cannot be 

 predicted to affect yield. 



