EFFECTS OF THE PAR LOCUS ON THE GROWTH-RATE AND STRUCTURAL STABILITY OF RECOMBINANT CELLS

A decreased growth rate of recombinant cells is observed when a cloned gene protein encoded in a multicopy plasmid is induced from a strong promoter. This negative effect on the cell growth rate may be the cons equence of alternate use or reallocation of energy, precursor metaboli tes, and protein synthesizing machinery. In order to analyze the cause s for this adverse effect, we have studied how genetic elements presen t in multicopy plasmids affect the growth of recombinant Escherichia c oli (K12DELTAH1DELTAtrpEA). Turning on of the P(L) promoter, whether o r not protein-coding sequences were present downstream of the promoter , decreased the growth of the recombinant cells by 15-50 %. This resul t suggests that the essential factor(s) for cell growth may be sequest ered by the genetic elements present in the plasmids and thus may caus e the decreased growth. The negative effect of the de-repressed P(L) p romoters on cell growth was partially reversed when the par sequence f rom pSC101 was inserted into the same plasmid. In each case when the p lasmid carried only the P(L) promoter, the P(L) promoter followed by t he N-terminal part of the protein-coding region, or the P(L) promoter followed by complete sequences encoding a protein, the par sequence ex hibited a positive effect on the growth rate of the recombinant cells, which usually grow at a slower rate than the host cell. In addition, it was found that the recombinant cells bearing the plasmid with the p ar locus were as resistant as the host cell to osmotic shock when expo sed to distilled water, whereas the recombinant cells bearing the plas mid without the par locus were sensitive to osmotic shock and resulted in lysis.