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Selective Isolation of Mutants - A majority of desirable mutants, especially the 'minor gene' mutants, showing increased production are isolated by screening a large number of clones surviving the mutagen treatment; this is called secondary screening. But this approach requires a large amount of work.

Therefore, efforts have increasingly focussed on developing techniques for the isolation of particular classes of mutants, which are likely to be overproducers.

1. Isolation of auxotrophic mutants is the basis for commercial amino acid production in Japan from the bacterium Corynebacterium glutamicus. For example, phe- mutants of C glutamicus accumulate tyrosine.

2. Many analogue-resistant mutants have feed-back insensitive enzymes of the biosynthetic pathway the analogue of whose product was used for selection of such cells. Such mutants tend to over­produce the end product of the concerned pathway.

3. Sometimes revertants from nonproducing mutants of a strain are high producers, e.g., one such reversion mutant of Streptomyces viridifaciens showed over 6-fold increase in chlortetracycline production over the original strain from which the nonproducing mutant was obtained.

4. Reversion mutants of appropriate auxotrophs may often be high producers.
5. In some cases, selection for resistance to the antibiotic produced by the organism itself may lead to increased yields.
6. Sometimes, mutants with altered cell membrane permeability show high production of some metabolites.
7. Mutants have been selected to produce altered metabolites, especially in case of aminogycoside antibiotics. For example, Pseudomonas aureofaciens produces the antibiotic pyrrolnitrin; a mutant of this fungus yields 4'- fluoropyrrolnitrin.

Mutant selection has been the most successful approach for strain improvement, but major advances are being made in the exploitation of other strategies, i.e., recombination and recombinant DNA technology.

 
 

 

 
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