Loss of Tdn catabolic genes by deletion from and curing of plasmid pTDN 1 in Pseudomonasputida : rate and mode of loss are substrate and pH dependent ?

The ability to degrade aromatic amines and m-toluate (Tdn+ phenotype), encoded by plasmid pTDN1, was lost from Pseudomonas put ih hosts after subculture in benzoate, succinate, acetate and glucose minimal medium, the fastest rate of loss occurring where benzoate was the substrate. Tdncells had either lost the entire pTDNl glasmid or suffered a recombinational deletion of a specific 26 kbp region. Proportional increase of Tdncells resulted from their growth-rate advantage, and additionally, where benzoate was the substrate. from its metabolism via the chromosomal ovtho-cleavage pathway incorporating a short lag phase. The ratio of whole plasmid loss to deletion was substrate and pH dependent. Deletion of catabolic genes was not required for loss of pTDNl but by comparison was a prerequisite for loss of TOL plasmid pWW0. It appeared that rn-toluate and benzoate were channelled via chromosomally encoded benzoate oxygenase and dihydroxycyclohexadiene carboxylate dehydrogenase prior to pTDN1 encoded meta-cleavage.