We have previously described a large family of mutants of Klebsiella aerogenes which were selected by continuous culture on xylitol and which superproduce ribitol dehydrogenase. One of these strains was found to harbour a high copy number 2·1 × 106 dalton plasmid. This plasmid is a deletion derivative of a low copy number 3·5 × 106 dalton plasmid present in the ancestral strain of K. aerogenes. However, since these plasmids do not contain the genes required for pentitol catabolism and some enzyme-superproducing strains have lost all DNA homologous to the plasmids, they are not implicated in the fast growth on xylitol. The plasmids contain regions of homology with the Escherichia coli plasmid ColE 1.
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