Summary: mt-2, harbouring the TOL plasmid pWW0, was grown in chemostat culture under succinate-, sulphate-, ammonium- or phosphate-limitation at different dilution rates. The fraction of mutant cells lacking the plasmid-encoded enzymes for the degradation of toluene and xylene (TOL cells), was determined. Genetic analysis revealed that all TOL cells isolated harboured partially deleted plasmids, lacking the TOL catabolic genes. The growth-rate advantage of the TOL cells was quantified from the kinetics of their increase as a fraction of the total population. At a dilution rate of 0.1 h no growth-rate advantage of TOL cells was found when phosphate or ammonium were limiting. Under sulphate-limitation, ingrowth of TOL cells was evident but did not follow a straightforward pattern. Under succinate-limitation the growth-rate advantage was the highest, particularly at low dilution rates (about 50% at = 0.05 h) In phauxostat culture, at the maximal growth rate, the growth-rate advantage of TOL cells was less than 1%. The specific activity in TOL cells of the plasmid-encoded enzyme catechol 2,3-dioxygenase was relatively high at a low growth rate.


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