Regulation oflac operon expression in mixed sugar chemostat cultures

Structured model of genetic control via the lac promoter in Escherichia coli

A model that describes induction of protein synthesis from lac-based promoters has been developed and incorporated into the single-cell model of Escherichia coli with transcriptional and translational modifications. Unlike previous models of lac-based promoters, this model allows a priori prediction of the intracellular parameters controlling transcription from lac-based promoters with only the extracellular levels of substrate and inducer as inputs. Because of the structural detail of the model, it is possible to simulate different genetic constructions for comparison, such as Lacl(q) strains versus wild-type cells, or including lacl on a multicopy plasmid. Expression from lac to tac promoters is predicted to yield 5% and 30% of the total cellular protein, respectively, with a pBR322-type plasmid. The model predicts the experimental observation that the Lacl(q) strain is not as fully induced as the wild-type strains, even at higher inducer concentrations. Additionally, the model predicts the right order of magnitude of protein production from lac and tac promoters when mechanisms for attenuation of transcription at lower translational efficiency are considered. Finally, the model predicts that for high copy number systems ribosomes become limiting in the synthesis of plasmid-encoded proteins. (c) 1994 John Wiley & Sons, Inc.

Overexpression of cloned genes using recombinant Escherichia coli regulated by a T7 promoter: II. Two-stage continuous cultures and model simulations

A two-stage culture strategy was studied for continuous high-level production of a foreign protein in the chemically inducible T7 expression system. The first stage is dedicated to the maintenance of plasmid-bearing cells and the second stage to the target protein synthesis by induction of cells coming from the first stage. On entering the second stage, recombinant cells undergo a gradual induction of the target gene expression. These plasmid-bearing cells experience dynamic changes in intracellular compositions and specific growth rates with their individual residence times. Therefore, the overall cultural characteristics in the production stage are really averages of the contributions from the various cells with different residence times. The behavior of the two-stage culture is described by a model, which accounts for dynamic variations of cell growth and protein synthesis rates with cell residence times. Model simulations were compared with experimental results at a variety of operating conditions such as inducer concentration and dilution rate. This model is useful for understanding the behavior of two-stage continuous cultures.

Comparison of deterministic and stochastic models of the lac operon genetic network

The lac operon has been a paradigm for genetic regulation with positive feedback, and several modeling studies have described its dynamics at various levels of detail. However, it has not yet been analyzed how stochasticity can enrich the system's behavior, creating effects that are not observed in the deterministic case. To address this problem we use a comparative approach. We develop a reaction network for the dynamics of the lac operon genetic switch and derive corresponding deterministic and stochastic models that incorporate biological details. We then analyze the effects of key biomolecular mechanisms, such as promoter strength and binding affinities, on the behavior of the models. No assumptions or approximations are made when building the models other than those utilized in the reaction network. Thus, we are able to carry out a meaningful comparison between the predictions of the two models to demonstrate genuine effects of stochasticity. Such a comparison reveals that in the presence of stochasticity, certain biomolecular mechanisms can profoundly influence the region where the system exhibits bistability, a key characteristic of the lac operon dynamics. For these cases, the temporal asymptotic behavior of the deterministic model remains unchanged, indicating a role of stochasticity in modulating the behavior of the system.

Transport regulation of recombinant gene expression in E. coli and B. subtilis

Expression kinetics of the lactose (lac) operon in Escherichia coli are reviewed for both wild-type and recombinant cell cultures under chemostatic conditions. A unified model which involves regulation of active inducer (lactose) transport, promoter-operator regulated expression of the lac operon, glucose-mediated inducer exclusion, and catabolite repression is summarized and supporting data is shown to verify its accuracy. The synthesis of alpha-amylase with a recombinant form of Bacillus subtilis is also reviewed to point out generic features in transport regulation, the lac operon model providing a point of departure. While there are many similarities in the influence of transport on both regulating models, there are also important differences. In a chemostat system, the synthesis of alpha-amylase is nongrowth associated, while beta-galactosidase is a growth-associated enzyme. Nevertheless, transport regulation is an important feature in both instances.