PLASMID MAINTENANCE IN ESCHERICHIA-COLI RECOMBINANT CULTURES IS DRAMATICALLY, STEADILY, AND SPECIFICALLY INFLUENCED BY FEATURES OF THE ENCODED PROTEINS

A set of eight closely related plasmid constructs carrying C1857-contr olled recombinant genes has been used as a model to study plasmid stab ility in Escherichia coli, in the absence of antibiotic selection. Pla smid loss rates and relative interdivision times of plasmid-bearing ce lls and plasmid-free cells have been analyzed throughout prolonged cul tures. Whereas the calculated plasmid loss rates are not consistent fo r a given plasmid and set of conditions, the relative growth fitness o f plasmid-bearing cells is highly reproducible. In the absence of gene expression, plasmid maintenance is influenced by the length of the cl oned segment, the growth temperature, and the plasmid copy number, but not by the plasmid size. At high, inducing temperatures, the effects of the metabolic burden are eclipsed by the toxicity exhibited by the different proteins produced, which is determined by structural feature s. Despite the multifactorial nature of the negative pressures acting independently on plasmid-bearing cells, the relative cell fitness in a mixed cell population is very reproducible for a given vector, result ing in a monotonous spread of the plasmid-free cells in recombinant cu ltures. (C) 1998 John Wiley & Sons, Inc.