Characterization of two plasmids arising spontaneously in phosphate-limited continuous cultures of Escherichia coli HB101[pAT153].

Two derivatives of plasmid pAT153 which arose spontaneously in phosphate-limited continuous cultures of Escherichia coli HB101[pAT153] have been characterised. The smaller plasmid, pLAB-446, resulted from a deletion of 446 base pairs of DNA covering the translation start and codon sequence of the tetracycline resistance gene. The other plasmid, pLAB135, differed from pAT153 only by an A:T to G:C transition in the transcribed but untranslated leader region of the tet gene which decreased tet transcription by 84%, possibly due to the formation of a more stable stem-loop structure in the mRNA.