Metabolic engineering of E. coli for pyocyanin production

Pyocyanin is a secondary metabolite from Pseudomonas aeruginosa that belongs to the class of phenazines, which are aromatic nitrogenous compounds with numerous biological functions. Besides its antifungal and antimicrobial activities, pyocyanin is a remarkable redox-active molecule with potential applications ranging from the pharma industry to the development of microbial fuel cells. Nevertheless, pyocyanin production has been restricted to P. aeruginosa strains, limiting its practical applicability. In this study, the pyocyanin biosynthetic pathway was engineered for the first time for high level production of this compound in a heterologous host. Escherichia coli cells harboring the nine-gene pathway divided into two plasmids were able to produce and secrete pyocyanin at higher levels than some Pseudomonas aeruginosa strains. The influence of culture and induction parameters were evaluated, and the optimized conditions led to an increase of 3.5-fold on pyocyanin accumulation. Pathway balancing was achieved by testing a set of plasmids with different copy numbers to optimize the expression levels of pyocyanin biosynthetic genes, resulting in a fourfold difference in product titer among the engineered strains. Further improvements were achieved by co-expression of Vitreoscilla hemoglobin Vhb, which relieved oxygen limitations and led to a final titer of 18.8 mg/L pyocyanin. These results show promise to use E. coli for phenazines production, and the engineered strain developed here has the potential to be used in electro-fermentation systems where pyocyanin plays a role as electron-shuttle.

The single-domain globin of Vitreoscilla: augmentation of aerobic metabolism for biotechnological applications

Extensive studies have revealed that large-scale, high-cell density bioreactor cultivations have significant impact on metabolic networks of oxygen-requiring production organisms. Oxygen transfer problems associated with fluid dynamics and inefficient mixing efficiencies result in oxygen gradients, which lead to reduced performance of the bioprocess, decreased product yields, and increased production costs. These problems can be partially alleviated by improving bioreactor configuration and setting, but significant improvements have been achieved by metabolic engineering methods, especially by heterologously expressing Vitreoscilla hemoglobin (VHb). Vast numbers of studies have been accumulating during the past 20 years showing the applicability of VHb to improve growth and product yields in a variety of industrially significant prokaryotic and eukaryotic hosts. The global view on the metabolism of globin-expressing Escherichia coli cells depicts increased energy generation, higher oxygen uptake rates, and a decrease in fermentative by-product excretion. Transcriptome and metabolic flux analysis clearly demonstrate the multidimensional influence of heterologous VHb on the expression of stationary phase-specific genes and on the regulation of cellular metabolic networks. The exact biochemical mechanisms by which VHb is able to improve the oxygen-limited growth remain poorly understood. The suggested mechanisms propose either the delivery of oxygen to the respiratory chain or the detoxification of reactive nitrogen species for the protection of cytochrome activity. The expression of VHb in E. coli bioreactor cultures is likely to assist bacterial growth through providing an increase in available intracellular oxygen, although to fully understand the exact role of VHb in vivo, further analysis will be required.

Intrinsic non-symbiotic and truncated haemoglobins and heterologous Vitreoscilla haemoglobin expression in plants

To date, haemoglobins (Hbs) have been shown to exist in all kingdoms of life. The least studied and understood groups are plant non-symbiotic haemoglobins (nsHbs) and the recently found plant truncated Hbs (trHbs). From a biotechnological point of view, the best characterized and almost exclusively applied Hb is the bacterial Vitreoscilla haemoglobin (VHb). In this review, the present state of knowledge of structural features and ligand binding kinetics of plant nsHbs and trHbs and their proposed roles as oxygen carriers, oxygen sensors, and for oxygen storage, in nitric oxide (NO) detoxification, and in peroxidase activity are described. Furthermore, in order to predict the functioning of plant Hbs, their characteristics will be compared with those of the better known bacterial globins. In this context, the effects of heterologous applications of VHb on plants are reviewed. Finally, the challenging future of plant Hb research is discussed.

An investigation of the peroxidase activity of Vitreoscilla hemoglobin

In order to investigate the ability of the Vitreoscilla hemoglobin (VHb) to act as a peroxidase, the protein was overexpressed in Escerichia coli and purified using a 6xHis-tag. The peroxidase activity of VHb was studied using 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), ferrocene carboxylic acid (FcCOOH) dopamine and L-dopa as substrates. The effects of external agents such as pH, salt concentration/ionic strength, and the thermal stability of VHb on the catalytic activity were assessed. The optimum pH for VHb using ABTS as a substrate was estimated to be 6-7. The VHb protein proved to be stable up to 80 degrees C, as judged by its peroxidase activity. Furthermore, NaCl concentrations up to 100 mM did not exert any significant effect on the activity. The catalytic activity against ABTS and FcCOOH was similar to that measured for horseradish peroxidase, whereas in the case of the phenolic substrates dopamine and L-dopa the activity was several orders of magnitude lower. The Michaelis constants, KmH2O2, were in good agreement with the data for human and bovine hemoglobin. No activity could be detected for the negative controls lacking VHb. These results demonstrate that VHb exhibits peroxidase activity, a finding in line with the hypothesis that VHb has cellular functions beyond the role as an oxygen carrier.

Improvement of bioremediation by Pseudomonas and Burkholderia by mutants of the Vitreoscilla hemoglobin gene (vgb) integrated into their chromosomes

Using genetic engineering, the Vitreoscilla (bacterial) hemoglobin gene (vgb) was integrated stably into the chromosomes of Pseudomonas aeruginosa and Burkholderia sp. strain DNT. This was done for both wild type vgb and two site-directed mutants of vgb that produce Vitreoscilla hemoglobin (VHb) with lowered oxygen affinities; in all cases functional VHb was expressed. Similar to previous results, the wild type VHb improved growth for both species and degradation of 2,4-dinitrotoluene (Burkholderia sp.) or benzoic acid (P. aeruginosa) under both normal and low aeration conditions. Both mutant vgbs enhanced these parameters compared to wild type vgb, and the improvement was seen in both species. The enhancements were generally greater at low aeration than at normal aeration. The results demonstrate the possibility that the positive effects provided by VHb may be augmented by protein engineering.

Gene expression profiling of Escherichia coli expressing double Vitreoscilla haemoglobin

In a recent investigation, expression of a double Vitreoscilla haemoglobin (two fused VHb molecules) in Escherichia coli grown in shake flasks resulted in higher final cell density and considerably higher levels of ribosomes and tRNA. In this study, we have investigated the E. coli transcriptome in cells expressing native VHb, double VHb and control cells lacking VHb by hybridising mRNA from the different constructs to high-density oligonucleotide arrays. Within the 95% confidence interval, 4 and 5% of all detected genes in native VHb cells were up- and down-regulated, respectively; in double VHb cells the corresponding numbers were 6 and 10%, respectively. Dividing the data into different functional groups revealed that genes involved in energy metabolism, central intermediary metabolism and cell processes were the most affected at the mRNA level. Particularly, the up-regulation of genes involved in translation and posttranslational modification observed in double VHb cells demonstrates a strong relationship between the regulation of ribosomal genes and the actual number of ribosomes.

Bacterial hemoglobins and flavohemoglobins: versatile proteins and their impact on microbiology and biotechnology

In response to oxygen limitation or oxidative and nitrosative stress, bacteria express three kinds of hemoglobin proteins: truncated hemoglobins (tr Hbs), hemoglobins (Hbs) and flavohemoglobins (flavo Hbs). The two latter groups share a high sequence homology and structural similarity in their globin domain. Flavohemoglobin proteins contain an additional reductase domain at their C-terminus and their expression is induced in the presence of reactive nitrogen and oxygen species. Flavohemoglobins detoxify NO in an aerobic process, termed nitric oxide dioxygenase reaction, which protects the host from various noxious nitrogen compounds. Only a small number of bacteria express hemoglobin proteins and the best studied of these is from Vitreoscilla sp. Vitreoscilla hemoglobin (VHb) has been expressed in various heterologous hosts under oxygen-limited conditions and has been shown to improve growth and productivity, rendering the protein interesting for biotechnology industry. The close interaction of VHb with the terminal oxidases has been shown and this interplay has been proposed to enhance respiratory activity and energy production by delivering oxygen, the ultimate result being an improvement in growth properties.

Time-minimized determination of ribosome and tRNA levels in bacterial cells using flow field-flow fractionation

The evaluation of the translation capacity of cells that produce recombinant proteins can be made by monitoring their ribosomal composition. In a previous use of asymmetrical flow field-flow fractionation (AsFlFFF) for this purpose the overall analysis time was more than 1 h and 40 min, based on a standard protocol for cell harvest, washing, cell disruption, and the final 8-min AsFlFFF determination of ribosome and subunits. In the present work the overall analysis time was reduced to 16 min. The washing step was deleted and a time-consuming freeze-thaw cycle. Cell disruption was obtained by a time-minimized lysozyme and detergent treatment for 1.5 min, respectively. The ribosomal material was finally fractionated and quantified in only 6 min, without previous centrifugation, using AsFlFFF. The great time reduction will enable the future use of AsFlFFF at-line to a growing cell cultivation, continuously monitoring the change in ribosomal composition or in other applications requiring high sample throughput. To demonstrate the high efficiency of the method the ribosome and tRNA composition in an Escherichia coli cultivation was monitored every half an hour, giving 18 measurements across the complete growth curve, a frequency of data enough to make decisions about induction or termination of the cultivation.

Microbial globins

Globins are an ancient and diverse superfamily of proteins. The globins of microorganisms were relatively ignored for many decades after their discovery by Warburg in the 1930s and rediscovery by Keilin in the 1950s. The relatively recent focus on them has been fuelled by recognition of their structural diversity and fine-tuning to fulfill (probably) discrete functions but particularly by the finding that a major role of certain globins is in protection from the stresses caused by exposure to nitric oxide (NO)--itself a molecule that has attracted intense curiosity recently. At least three classes of microbial globin are recognised, all having features of the classical globin protein fold. The first class is typified by the myoglobin-like haemprotein Vgb from the bacterium Vitreoscilla, which has attracted considerable attention because of its ability to improve growth and metabolism for biotechnological gain in a variety of host cells, even though its physiological function is not fully understood. The truncated globins are widely distributed in bacteria, microbial eukaryotes as well as plants and are characterised by being 20-40 residues shorter than Vgb. The polypeptide is folded into a two-over-two helical structure while retaining the essential features of the globin superfamily. Roles in oxygen and NO metabolism have been proposed. The third and best understood class comprises the flavohaemoglobins, which were first discovered and partly characterised in yeast. These are distinguished by the presence of an additional domain with binding sites for FAD and NAD(P)H. Widely distributed in bacteria, these proteins undoubtedly confer protection from NO and nitrosative stresses, probably by direct consumption of NO. However, a bewildering array of enzymatic capabilities and the presence of an active site in the haem pocket reminiscent of peroxidases hint at other functions. A full understanding of microbial globins promises advances in controlling the interactions of pathogenic bacteria with their animal and plant hosts, and manipulations of microbial oxygen transfer with biotechnological applications.

A memorial review of Jay Bailey's contribution in prokaryotic metabolic engineering

When mentioning prokaryotic metabolic engineering, most people will immediately think of Jay Bailey. Jay's contribution to this fast-growing field is evident and familiar to many. Therefore, instead of a detailed technical review, we attempt in this article to summarize his contribution and dissect reasons for his success in this area from a standpoint of one of his former students (VH) and of a colleague in the field (JCL). This short review is by no means complete and provides only a partial view of Jay's contribution to the metabolic engineering of prokaryotes.

Recombinant protein expression for therapeutic applications

In recent years, the number of recombinant proteins used for therapeutic applications has increased dramatically. Many of these applications involve complex glycoproteins and antibodies with relatively high production needs. These demands have driven the development of a variety of improvements in protein expression technology, particularly involving mammalian and microbial culture systems.

Expressing creatine kinase in transgenic tobacco--a first step towards introducing an energy buffering system in plants

Creatine kinase a key enzyme in cellular energy homeostasis of vertebrates offers the promise of engineering plants with enhanced stress tolerance. In order to provide plants with such an energy buffering system, tobacco was transformed with a cDNA, encoding the cytosolic brain-type isoform of chicken creatine kinase (BB-CK), the expression of which was under the control of the cauliflower mosaic virus 35S (CaMV 35S) promoter. Transgenic tobacco plants were selected and suspension cultures generated. Both transgenic plants and suspension cultures were shown to stably express enzymatically active BB-CK in vitro and in vivo, and in most cases for three successive generations (T0-T2). Exogenously supplied creatine was shown to enter the plant cells and resulted in only a slight reduction in root growth at concentrations up to 10 mM. Furthermore, the BB-CK expressing tobacco plants and cell suspension cultures were able to convert creatine into phosphocreatine.

Targeted expression of a synthetic codon optimized gene, encoding the spruce budworm antifreeze protein, leads to accumulation of antifreeze activity in the apoplasts of transgenic tobacco

A synthetic gene based on the primary sequence of the mature spruce budworm antifreeze protein (sbwAFP) was constructed by primer overlap extension. The amino acid codons were chosen to mimic those of a highly expressed tobacco nuclear gene. A DNA sequence encoding the amino-terminal leader sequence from the tobacco pathogen related protein 1b (PR), which targets the protein to the apoplastic space, was fused in frame to the synthetic sbwAFP gene. This fusion was placed downstream of the cauliflower mosaic virus 35S promoter and upstream of the nopaline synthase terminator in a T-DNA binary vector. Transgenic tobacco lines transcribing PR-sbwAFP were selected by RT-PCR. The apoplastic protein fractions of sbwAFP expressing tobacco lines exhibited enhanced antifreeze activity as demonstrated by the ability to inhibit ice re-crystallization and increased thermal hysteresis.