Dual control of invertase biosynthesis in chemostat culture

Production of extracellular α-amylase by single-stage steady-state continuous cultures of Candida wangnamkhiaoensis in an airlift bioreactor

The kinetics of growth and α-amylase production of a novel Candida wangnamkhiaoensis yeast strain were studied in single-stage steady-state continuous cultures. This was performed in a split-cylinder internal-loop airlift bioreactor, using a variety of carbon sources as fermentation substrates. Results showed that the steady-state yields of cell mass from carbohydrates were practically constant for the range of dilution rates assayed, equaling 0.535 ± 0.030, 0.456 ± 0.033, and 0.491 ± 0.035 g biomass/g carbohydrate, when glucose, maltose, and starch, respectively were used as carbon sources. No α-amylase activity was detected when glucose was used as the carbon source in the influent medium, indicating that α-amylase synthesis of C. wangnamkhiaoensis is catabolically repressed by glucose. Contrastingly, maltose and starch induce synthesis of α-amylase in C. wangnamkhiaoensis, with starch being the best α-amylase inducer. The highest α-amylase volumetric and specific activities (58400 ± 800 U/L and 16900 ± 200 U/g biomass, respectively), and productivities (14000 ± 200 U/L·h and 4050 ± 60 U/g biomass·h, respectively) were achieved at a dilution rate of 0.24 h-1 using starch as the carbon source. In conclusion, single-stage steady-state continuous culture in an airlift bioreactor represents a powerful tool, both for studying the regulatory mechanisms of α-amylase synthesis by C. wangnamkhiaoensis and for α-amylase production. Furthermore, results showed that C. wangnamkhiaoensis represents a potential yeast species for the biotechnological production of α-amylase, which can be used for the saccharification of starch. This offers an attractive renewable resource for the production of biofuels (particularly bioethanol), representing an alternative to fossil fuels with reduced cost of substrates.

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.

Dynamics of induced CAT expression in E. coli

The dynamics of chemically induced chloramphenicolaceyl-transferase (CAT) expression are determined in batch cultures of Escherichia coli DH5alphaF'IQ [pKK262-1]. This article is directed towards understanding the coupling of induced cloned-protein synthesis and reduced cell growth which are balanced in the optimal system. Experimental results indicate that the best inducer (IPTG) concentration is near 1.0 mM when added during midexponential growth. Lower concentrations cause only weak induction, whereas higher concentrations cause sufficiently strong induction that cell growth is suppressed. Induction at the onset of the stationary phase results in high expression but is accompanied by stimulated protease activity. Also, cell mass yield is adversely affected by enhanced protein synthesis. A structured metabolic model is shown to predict the responses of instantaneous growth rate and productivity which result from protein overexpression. This model can be employed to predict alternative reactor strategies and operating conditions necessary for the design of efficient bioprocess.

Production of biomass and beta-D-galactosidase by Candida pseudotropicalis grown in continuous culture on whey

The production of biomass and beta-D-galactosidase by the lactose-utilizing yeast Candida pseudotropicalis NCYC 744 in whey medium was studied. Apparent optimization of growth conditions and medium was done in continuous culture. Optimal pH and temperature were 2.6 and 36-38 degrees C, respectively, Limitations in Cu, Zn, and possibly Mn were detected in deproteinized whey medium. Additions of tryptophan stimulated growth of the yeast. Under optimal conditions in medium supplemented with excess tryptophan, Cu, Zn, and Mn the maximum values obtained: yeast concentration, 4.6 g/L; yeast productivity, 1.4 g/L h (at D = 0.35 h(-1)); enzyme volumetric productivity, 2100 U/L h (at D = 0.25 h(-1)); maintenance coefficient, 5-10 mg lactose/g cell h; saturation constant (K(s)) for lactose, 4.76mM; maximum specific growth rate, (mu(max)), 0.47 h(-1). No significant increase in specific enzyme activity (U/mg cell) was observed after medium optimization evidencing the importance of regulatory controls in enzyme synthesis.

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.

Theoretical and methodological studies of continuous microbial bioreactors

This article reviews most of the author's studies on process development and reactor design for continuous microbial reactions. (1) Enzyme reactions of growing and non-growing microbial cells immobilized in agar gel beads were analyzed pertaining to the effects of external and internal diffusion of substrate on reaction kinetics. (2) Experimental correlations of production rates of beta-fructosidase and acid phosphatase with dilution rate of continuous culture were simulated based on an operon model for enzyme regulation. (3) Population dynamics of an amylase-producing bacteria and their mutant were discussed in relation to enzyme productivity in a continuous culture of spore-forming bacteria. (4) Plasmid mobilization in a mixed population of donor, recipient, and helper cells was investigated in a continuous culture as a model study of accidental release of a genetically modified plasmid into a natural environment. (5) A production rate increase of up to 100-fold was achieved by cell-recycle culturing of continuous acetic acid fermentation using a filter module with a hollow fiber membrane. (6) The feasibility of a continuous surface culture for the biooxidation of organic substances was ascribed to an enhanced oxygen absorption rate in the presence of a microbial film on a liquid surface. (7) Simultaneous separation of inhibitory products using an electrodialysis module during some organic acid fermentations was effective for increasing production in a continuous culture.

Regulation of beta-exoglucanase activity production by Saccharomyces cerevisiae in batch and continuous culture

The rate of synthesis and secretion of exo-1-3-beta-glucanase activity closely paralleled the specific rate of growth in exponentially growing Saccharomyces cerevisiae cells in batch culture. When the stationary phase was reached both synthesis and secretion stopped. No activity was synthesized when the cells were maintained in carbon sources that did not allow them to grow. Studies in continuous culture indicate a strong relationship between the synthesis of exoglucanase activity and the specific growth rate. These results are taken as evidence of an essential role of this activity during the yeast budding cycle.

Kinetics of product formation and plasmid segregation in recombinant microbial populations

Experimental and mathematical analysis of productivity of cultures containing recombinant plasmids is based in this work on the following paradigm: molecular controls leads to single cell kinetics leads to cell population dynamics leads to reactor productivity. Mathematical models have been developed for replication control of the lambda dv plasmid and for efficiency of the lac promoter-operator based on the molecular control mechanisms of these systems in Escherichia coli. A special and important attribute of these models is their ability to describe quantitatively a wide range of genetic effects as well as environmental influences on molecular control function. Equations describing plasmid maintenance and distribution in growing cell cultures have been determined based on the population-balance equations applicable to a segregated culture model. The washout dilution rate for continuous cultivation of plasmid-containing Saccharomyces cerevisiae in selective medium is given in terms of single-cell division-cycle parameters and plasmid copy number for one single-cell model of plasmid replication and segregation. A new experimental method based on flow cytometry for rapid characterization of heterogeneity of single-cell accumulation of a plasmid gene product is also described. Generalization of these methods and of the overall strategy should provide a useful framework for synthesis of biological and engineering principles and methods to optimize organisms and processes based on recombinant DNA technology.

Production of extracellular enzymes in continuous culture

The production of bacterial enzymes in batch fermentations is compared with results obtained in continuous culture. When studying the production of alpha-amylase in Bacillus subtilis it was found that instability of the enzyme synthesis was due to nonhomogeneity of the population rather than to "the culture's history" (i.e. succession of several physiological states necessary for the enzyme production). The plasmid contained in the production clone was found to be the factor responsible for the alpha-amylase production. Predominance of the production clone or of the nonproduction one depends on the cultivation conditions used. As compared with batch cultivation the continuous production yields higher enzyme concentrations under optimal conditions and the fermentor productivity may be four to five times higher.

Production of Maltase by Wild-Type and a Constitutive Mutant of Saccharomyces italicus

The production of maltase, an inducible and repressible catabolic enzyme in Saccharomyces italicus , was studied and compared in batch, fed-batch, and continuous fermentations. Tight genetic controls on maltase synthesis limited the effect of environmental manipulations such as fed-batch or continuous culture in enhancement of maltase synthesis, and neither approach was able to improve the performance above the batch process for maltase production. S. italicus was mutated, and a constitutive producer of maltase was isolated. The mutant was detected by its ability to grow on sucrose, which is a noninducing substrate that is hydrolyzed by maltase; S. italicus does not possess invertase and will not normally grow on sucrose. Maltase production by this mutant was studied during growth on sucrose in batch and continuous cultures and marked improvement in enzyme productivity was observed. The specific activity of maltase produced by this mutant was more than twice that of the parent wild type: 2,210 and 1,370 U/g of cells for the mutant versus 890 and 510 U/g of cells for the wild type in batch and continuous cultures, respectively. Maltase specific productivity was increased from 74 to 288 U/g of cells per h by switching from batch growth of the wild type to continuous cultivation of the mutant.

Mathematical model of cell growth and phosphatase biosynthesis in Saccharomyces carlsbergensis under phosphate limitation

The rate kinetics of growth and acid phosphate formation in the batch culture of Saccharomyces carlsbergensis LAM 1068 was studied under varying degrees of phosphate limitation. The mathematical model that was developed is concerned with the time lag for exponential growth, the biphasic growth on a substrate (glucose) and its product (ethanol), sustained growth on conservative phosphate, and the derepression of acid phosphatase. The numerical calculations using appropriate parametric constants successfully described the variation in the cell mass, glucose, ethanol, and inorganic phosphate concentrations, and the enzyme activity of acid phosphatase during aerobic growth of S. carlsbergensis under five different conditions of phosphate starvation. A simulation study revealed that the optimum initial phosphate concentration in the medium giving a high productivity of acid phosphatase was 2.0 mg phosphorus/g glucose liter.

Modeling the role of cyclic AMP in catabolite repression of inducible enzyme biosynthesis in microbial cells

Modeling the role of cyclic AMP (cAMP) in catabolite repression of inducible enzyme production in microbial cells was studied. A catabolite repression index, F, was defined based on the postulation that complex formation occurs between RNA polymerase (RNAP) and DNA, and shifting from the inert form to the open form of this complex (the latter form is required for transcription) is accelerated by the cAMP.CRP complex. The catabolite repression index, F, was incorporated into model equations of mRNA production. Empirical relationships between intracellular cAMP level and medium glucose concentration were established based on experimental data and introduced into the model. Computer simulation results were obtained for a number of interesting cases. The practical utility of the proposed model was demonstrated by comparing it with the experimental results on glucose isomerase biosynthesis.