Stabilization by the Mini-F Fragment of a pBR322 Derivative Bearing the Tryptophan Operon in Escherichia coli

Abstract

In an Escherichia coli K-12 strain (trpA trpE tnd) cultured in LB broth without selective pressure, a pBR322 derivative bearing the E. coli tryptophan Operon (pBR322-trp) was rapidly lost: after 27 cell-number doublings, only 7% cells retained both tryptophan prototrophy (Trp⁺) and ampicillin resistance (Ap^r), and 17% were Ap^r but Trp⁻. Insertion of the mini-F DNA from F factor into this plasmid effectively suppressed both the plasmid loss and the discoordinate loss of Trp⁺: the percentage of Trp⁻ cells per cell-number doubling was decreased more than 100-fold. Partial derepression of the trp operon due to 3-indole acrylic acid further decreased the stability of the pBR322-trp but not that of the mini-F-inserted pBR322-trp.