Summary: Recurrent mutation and selection has been used to increase penicillin titre in two closely related strains of A selection programme was initiated from each of the two strains (programmes A and B) and continued through six cycles of mutation and selection. Near-ultraviolet light in conjuction with 8-methoxypsoralen was employed as the sole mutagen throughout programme A and ethyl methanesulphonate as the sole mutagen throughout programme B. Excluding the first cycle of A, where only 50 strains were assayed, the selection programmes were identical. In each programme, 100 survivors were assayed for penicillin titre after each mutagenic treatment and, on the basis of a single yield test, the best five strains were picked and carried forward to the next cycle. In both selection programmes, a near 300% increase in penicillin titre was achieved. This yield advance illustrates the effectiveness for strain development of experimental designs involving successive cycles of mutagenesis with a single-stage screen and the selection of the top few percent survivors in each cycle.


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