nelson: screening methods in microbiology 319 



Excluding a direct link between the specific block in the auxo- 

 troph and the antibiotic synthesized, is there any reason to suspect a 

 change in yield? Continued improvements in yield have been con- 

 sidered to result from mutations preventing diversion of energy and 

 substrate into side reactions (41). Thus, the more nutritional defi- 

 ciencies introduced into a strain, the higher the expected yield, 

 providing the fermentation medium is supplied with a sufficiency of 

 the required nutrients. A corallary of this hypothesis is the prediction 

 that strains with high yields are auxotrophs. No significantly 

 improved yields have been found among auxotrophs. However, the 

 levels of loss of biochemical function which affect antibiotic synthesis 

 may be more subtle than those existing in all-or-none auxotrophs. 



Another line of reasoning predicting yield increases in auxo- 

 trophs follows from the suggestion that a medium with a surplus of 

 energy sources but limiting in nutrients essential to growth, but not 

 antibiotic synthesis, results in antibiotic production (61). Thus, anti- 

 biotic production can be viewed as a consequence of unbalanced 

 growth (33). While phosphate is usually the growth limiting factor, a 

 lack of phosphate may depress the rate of production of high energy 

 intermediates and limit antibiotic production as well as the level of 

 growth. Furthermore, continuing ribose nucleic acid synthesis may 

 be a prerequisite for enzyme synthesis and maintenance. Limiting 

 phosphate would adversely affect both energy generation and enzyme 

 synthesis. A specific method for blocking deoxyribose nucleic acid 

 synthesis is required. Physical (ultraviolet irradiation) or chemical 

 (addition of pyrimidine analogs or transmethylation inhibitors) 

 methods are too cumbersome for industrial use. A thymine-requiring 

 auxotroph would have a built-in method for growth level adjust- 

 ment by control of the thymine concentration of the medium. 



Mutants blocked in alternate pathways of dissimilation and 

 energy generation, such as the "poky" strains of Neurospora (58), 

 would be expected to have changed rates of synthesis. Strains resistant 

 to acridine dyes, which may clear the cell of plasmid-like portions of 

 the cytochrome system (47), proved to have lower growth rates and 

 antibiotic synthesis. 



Mutagenic Techniques 



There are no clues to suggest that one mutagen is more effective 

 than another in producing variants with improved yield. The action 



