(A) study on the dynamic behaviours of a recombinant E. coli cell and stable maintenance of a recombinant plasmid in chemostat culture

Abstract

For the analysis of the dynamic behaviour of population and plasmid instability in recombinant DNA cell fermentation system, a chemostat culture was performed using a mutant of E. coli \3110 Δtrp LD $rp R^{ts}$ $tna^-$/P CRT 185 harboring a whole trp-operon. The specific growth rates of plasmid-harboring and plasmid-free host cells were $0.48 hr^{-1}$ and $0.59 hr^{-1}$ at the derepressed condition (42℃), respectively. The relationship between the specific production rate of tryptophan, qp, and the specific growth rate, μ, was parabolic; at $μ = 0.25 hr^{-1}$, qp was about 3.8mg/g-cell hr, and at $μ = 0.075 hr^{-1}$ qp was about 10.3mg/g-cell hr. The plasmid DNA content per g-cell was observed to increased from 0.2 to 0.5 mg per g-cell as dilution rate decreased from 0.425 to $0.075 hr^{-1}$. At dilution rate of $0.25 hr^{-1}$ and repressed condition (37℃), plasmid stability is stably maintained for 225 hr. When tryptophan was added, however, plasmid stability was oscillated. At derepressed condition (42℃), the number of plasmid harboring cells decreased due to the plasmid instability as culture time elapsed, meanwhile the number of plasmid-free cells was increased. When tryptophan concentration in culture broth was reached a zero level, plasmid free cells could not grow. The cells, consequently, was washed out. On the contrary, when tryptophan ($1×10^{-3}M$ was continuously added to the culture medium, culture broth was dominant with plasmid-free cells after 200 hrs and the cell concentration reached at steady state. A two-stage continuous culture coupled with the cell growth in first stage at 37℃and the gene expression in second stage at 42℃, enhanced significantly the plasmid stability (above 96%) and maintained almost constantly the specific production rate of tryptophan for more than 500 hrs.