Synthetic genetic programs promise to enable novel applications in industrial processes. For such applications, the genetic circuits that compose programs will require fidelity in varying and complex environments. In this work, we report the performance of two synthetic circuits in Escherichia coli under industrially relevant conditions, including the selection of media, strain, and growth rate. We test and compare two transcriptional circuits: an AND and a NOR gate. In E. coli DH10B, the AND gate is inactive in minimal media; activity can be rescued by supplementing the media and transferring the gate into the industrial strain E. coli DS68637 where normal function is observed in minimal media. In contrast, the NOR gate is robust to media composition and functions similarly in both strains. The AND gate is evaluated at three stages of early scale-up: 100 ml shake-flask experiments, a 1 ml MTP microreactor, and a 10 L bioreactor. A reference plasmid that constitutively produces a GFP reporter is used to make comparisons of circuit performance across conditions. The AND gate function is quantitatively different at each scale. The output deteriorates late in fermentation after the shift from exponential to constant feed rates, which induces rapid resource depletion and changes in growth rate. In addition, one of the output states of the AND gate failed in the bioreactor, effectively making it only responsive to a single input. Finally, cells carrying the AND gate show considerably less accumulation of biomass. Overall, these results highlight challenges and suggest modified strategies for developing and characterizing genetic circuits that function reliably during fermentation.

Directed evolution was performed on vanadium chloroperoxidase from the fungus Curvularia inaequalis to increase its brominating activity at a mildly alkaline pH for industrial and synthetic applications and to further understand its mechanism. After successful expression of the enzyme in Escherichia coli, two rounds of screening and selection, saturation mutagenesis of a "hot spot," and rational recombination, a triple mutant (P395D/L241V/T343A) was obtained that showed a 100-fold increase in activity at pH 8 (k cat ‫؍‬ 100 s ؊1 ). The increased K m values for Br ؊ (3.1 mM) and H 2 O 2 (16 M) are smaller than those found for vanadium bromoperoxidases that are reasonably active at this pH. In addition the brominating activity at pH 5 was increased by a factor of 6 (k cat ‫؍‬ 575 s ؊1 ), and the chlorinating activity at pH 5 was increased by a factor of 2 (k cat ‫؍‬ 36 s ؊1 ), yielding the "best" vanadium haloperoxidase known thus far. The mutations are in the first and second coordination sphere of the vanadate cofactor, and the catalytic effects suggest that fine tuning of residues Lys-353 and Phe-397, along with addition of negative charge or removal of positive charge near one of the vanadate oxygens, is very important. Lys-353 and Phe-397 were previously assigned to be essential in peroxide activation and halide binding. Analysis of the catalytic parameters of the mutant vanadium bromoperoxidase from the seaweed Ascophyllum nodosum also adds fuel to the discussion regarding factors governing the halide specificity of vanadium haloperoxidases. This study presents the first example of directed evolution of a vanadium enzyme.Haloperoxidases catalyze the oxidation of halides to hypohalous acids (see Equation 1), an industrially interesting reaction; these enzymes can be used to halogenate various organic compounds (1, 2) (see Equation 2). In addition they may provide an alternative biocide in antifouling applications (3-5) or may be used as a component in disinfectants and in detergent formulations for bleaching purposes (6 -8). Equations 1 and 2 illustrate the reactions occuring.where X is clorine, bromine, or iodine, and A is an organic nucleophilic acceptor.Presently two classes of enzymes are known that efficiently catalyze the oxidation of a halide by hydrogen peroxide, the vanadium haloperoxidases and the heme peroxidases. The heme peroxidases have a significant disadvantage of rapid inactivation during turnover because of an oxidative reaction of the heme group with the peroxide substrate and the formed hypohalous acids. Vanadium haloperoxidases (VHPOs), 2 which contain vanadate (VO 4 3Ϫ ) as a prosthetic group, do not suffer from this disadvantage (9); in addition they are much more resistant toward heat, detergent, and solvent denaturation (10). The major drawback of all haloperoxidases including the stable VHPOs is that they are mainly active at mildly acidic pH values, whereas for many applications activity at mildly alkaline pH values is required. An example of this is their potential use as an antif...

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Synthetic biology has developed numerous parts for the precise control of protein expression. However, relatively little is known about the burden these place on a host, or their reliability under varying environmental conditions. To address this, we made use of synthetic transcriptional and translational elements to create a combinatorial library of constructs that modulated expression strength of a green fluorescent protein. Combining this library with a microbioreactor platform, we were able to perform a detailed large-scale assessment of transient expression and growth characteristics of two Escherichia coli strains across several temperatures. This revealed significant differences in the robustness of both strains to differing types of protein expression, and a complex response of transcriptional and translational elements to differing temperatures. This study supports the development of reliable synthetic biological systems capable of working across different hosts and environmental contexts. Plasmids developed during this work have been made publicly available to act as a reference set for future research.