The recombinant plasmids RSF2124-trp and pSC101-trp were examined for their phenotypic stability in W3110 and its derivatives under various culture conditions. RSF2124-trp and pSC101-trp were stable in a strain. In an amber mutant of the tryptophan repressor gene, RSF2124-trp was fairly stable, whereas pSC101-trp was unstable. All Trp segregants from the pSC101-trp carrier had lost the entire plasmid. In a mutant carrying the mutation, RSF2124-trp was unstable in rich media. Most Trp segregants that appeared under these conditions were deleted in genes as well as in the gene on the recombinant plasmid. pSC101-trp in this mutant was also unstable. All Trp segregants had lost the plasmid. Studies of enzyme activities revealed that the greater the activity of anthranilate synthase and tryptophan synthase in bacteria, the more segregants tended to appear in the stability test. RSF2124 and pSC101 without the gene were completely stable in the same bacteria. The apparent instability of bacteria carrying the recombinant plasmid could be explained by the lower growth rate compared with bacteria carrying only the vector plasmid, resulting in the enrichment of Trp bacteria during culture.


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