nelson: screening methods in microbiology 321 



the organism and the technical competence of the worker. The possi- 

 bility of improved yield through recombination has been recognized 

 in a patent (84) but no examples of increased yield have been 

 reported. An attempt to improve yields of organic acids by the use 

 of heterokaryons did not succeed (29). Parasexual recombination in 

 penicillin-producing strains demonstrated the genetic control of 

 penicillin synthesis but did not result in yield improvement (28). 

 A diploid recombinant between variants of the Wisconsin family of 

 Penicillia separated by several mutational steps was reported to give 

 a 50 per cent yield improvement (93, 94). 



The discovery of recombination between actinomycetes leading 

 to heterokaryons and stable recombinants (22, 96) opened the way 

 to possible industrial application of such techniques to antibiotic- 

 producing microorganisms. A symposium on the genetics of actinomy- 

 cetes contains further papers (104). The formation of antibiotic- 

 producing recombinants from nonproducing mutants has been 

 described (21). In summary, these and other papers (4, 15, 16, 18, 19, 

 20, 25, 63, 89, 97) report the possibility of forming heterokaryons 

 within but not between "species" of streptomycetes and the occasional 

 strain-specific formation of stable recombinants, but no recovery of 

 industrially useful derivatives. 



This work on genetic interaction among actinomycetes provides 

 the basis for the possible production of isolates with increased anti- 

 biotic yield obtained as recombinants between strains of diverse 

 origin. Antibiotic production is probably dependent upon a complex 

 of genetically separable factors, affecting an estimated 30 different 

 biosynthetic steps through their enzymes and control systems. Recom- 

 binants obtained between genetically different strains may give 

 higher yields than either parental strain by reassortment of factors 

 limiting and enhancing these individual biosynthetic steps. An essen- 

 tial character of the strains selected for this analysis, other than the 

 necessity for the production of the same antibiotic, is divergent 

 geographic origin, ecology, and physiology. Recombinants derived 

 from mutants selected from the same line, of similar "pedigree", would 

 not be expected to have an enhanced yield and no increases with 

 measures of significance have been reported for such recombinants 

 (4, 93). Changes in ploidy within the same strain could affect yield 

 without the necessity of "outcrossing" (95). 



