Optimization of tryptophan production in bacteria. Design of a strategy for genetic manipulation of the tryptophan operon for tryptophan flux maximization.

In the present work we have applied the indirect optimization method (Torres, N. V. et al. Biotechnol. Bioeng. 1996, 49, 247-258) to the maximization of tryptophan biosynthesis in Escherichia coli. The optimization procedure is applied to an updated model of this biochemical system (Xiu, Z-L et al., J. Biotechnol. 1997, 58, 125-140) and thus extended to a problem that includes the processes of transcription and translation. The model representation used by these authors is first translated into the corresponding S-system version. Then, to guarantee cell viability, we impose a set of constraints on some variable and parameter values, all of which are able to be modulated by available techniques. Our results show that it is possible to attain a stable and robust steady state with a rate of tryptophan production increased more than 4 times. This is achieved by changing four key parameters related to the efflux of tryptophan, the growth rate, the inhibition constant, and the tryptophan repressor level. Moreover, it is demonstrated that we can reach this optimum state in a sequential manner, each step leading us to a better situation in relation to the previous one. Thus, only by doubling the tryptophan excretion we can triplicate the rate of tryptophan production. A further, although lesser, improvement can be attained by increasing 4-fold the rate of growth and subsequently by weakening the inhibitory feedback interaction of tryptophan on the enzymes leading to its synthesis. Finally, a significant jump in the rate of production can be obtained if the level of the trp operon could be decreased. When a second approach was considered, in which the growth rate is kept constant in the optimized profile, we found out that by modulation of the parameters it is possible to increase more than 2-fold the rate of tryptophan production.