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Summary: Pseudomonas putida mt-2, harbouring the TOL plasmid PWW0, was grown continuously on benzoate in a phauxostat at a non-limited rate. The gradual decrease in the population carrying the complete TOL plasmid was caused predominantly by a growth-rate advantage of spontaneous mutants carrying a partially deleted plasmid (TOL− cells). The growth-rate difference (v) was quantified both by measuring the increase in the dilution rate (from 0.68 to 0.79 h−1; v = 0.11 h−1) and by mathematical analysis of the ingrowth of TOL− cells (v = 0.12 h−1). The latter procedure also established that the segregation rate was of the order of magnitude 10−5 h−1. Similar values for the growth-rate advantage and the segregation rate were found when both benzoate and succinate were present in non-limiting concentrations. It is suggested that the growth-rate disadvantage of the wild-type strain is caused by inhibitory effects of an intermediate in the degradation of benzoate via the plasmid-encoded meta-pathway.
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