Optimization of recombinant human nerve growth factor production in the psychrophilic Pseudoalteromonas haloplanktis

Recombinant protein production in Pseudoalteromonas haloplanktis TAC125 biofilm

Biofilms have great potential for producing valuable products, and recent research has been performed on biofilms for the production of compounds with biotechnological and industrial relevance. However, the production of recombinant proteins using this system is still limited. The recombinant protein production in microbial hosts is a well-established technology and a variety of expression systems are available. Nevertheless, the production of some recombinant proteins can result in proteolyzed, insoluble, and non-functional forms, therefore it is necessary to start the exploration of non-conventional production systems that, in the future, could be helpful to produce some "difficult" proteins. Non-conventional production systems can be based on the use of alternative hosts and/or on non-conventional ways to grow recombinant cells. In this paper, the use of the Antarctic marine bacterium Pseudoalteromonas haloplanktis TAC125 grown in biofilm conditions was explored to produce two fluorescent proteins, GFP and mScarlet. The best conditions for the production were identified by working on media composition, and induction conditions, and by building a new expression vector suitable for the biofilm conditions. Results reported demonstrated that the optimized system for the recombinant protein production in biofilm, although it takes longer than planktonic production, has the same potentiality as the classical planktonic approach with additional advantages since it needs a lower concentration of the carbon sources and doesn't require antibiotic addition. Moreover, in the case of mScarlet, the production in biofilm outperforms the planktonic system in terms of a better quality of the recombinant product.

Expression and characterization of a lipase EstA from Bacillus subtilis KM-BS for application in bio-hydrolysis of waste cooking oil

A lipase EstA from Bacillus subtilis KM-BS was expressed in Escherichia coli BL21 (DE3) cells. The recombinant enzyme achieved high activity (49.67 U/mL) with protein concentration of 1.29 mg/mL under optimal conditions at the large-scale expression of 6 h and post-induction time at 30 °C using 0.1 mM isopropyl-β-d-thiogalactopyranoside (IPTG). The optimal temperature and pH of the purified enzyme were at 45-55 °C and pH 8.0 - 9.0, respectively. Activity of the purified enzyme was stable in the presence of 1 mM Ca2+; stimulated by 1 mM Mg2+ and Mn2+, and inhibited by Fe3+. A significant amount of fatty acids was released during the hydrolysis of waste cooking oil under the catalysis of purified lipase, indicating that this recombinant lipase showed promise as a suitable candidate in industrial fields, particularly in biodiesel and detergent sector.

Meeting report of the sixth annual tri-service microbiome consortium symposium

The Tri-Service Microbiome Consortium (TSMC) was founded to enhance collaboration, coordination, and communication of microbiome research among DoD organizations and to facilitate resource, material and information sharing amongst consortium members, which includes collaborators in academia and industry. The 6th Annual TSMC Symposium was a hybrid meeting held in Fairlee, Vermont on 27-28 September 2022 with presentations and discussions centered on microbiome-related topics within seven broad thematic areas: (1) Human Microbiomes: Stress Response; (2) Microbiome Analysis & Surveillance; (3) Human Microbiomes Enablers & Engineering; (4) Human Microbiomes: Countermeasures; (5) Human Microbiomes Discovery - Earth & Space; (6) Environmental Micro & Myco-biome; and (7) Environmental Microbiome Analysis & Engineering. Collectively, the symposium provided an update on the scope of current DoD microbiome research efforts, highlighted innovative research being done in academia and industry that can be leveraged by the DoD, and fostered collaborative opportunities. This report summarizes the activities and outcomes from the 6th annual TSMC symposium.

Development of high-copy number plasmids in Pseudoalteromonas haloplanktis TAC125

The Antarctic bacterium Pseudoalteromonas haloplanktis TAC125 (PhTAC125) is considered an interesting alternative host for the recombinant protein production, that can be explored when the conventional bacterial expression systems fail. Indeed, the manufacture of all the difficult-to-express proteins produced so far in this bacterial platform gave back soluble and active products. Despite these promising results, the low yield of recombinant protein production achieved is hampering the wider and industrial exploitation of this psychrophilic cell factory. All the expression plasmids developed so far in PhTAC125 are based on the origin of replication of the endogenous pMtBL plasmid and are maintained at a very low copy number. In this work, we set up an experimental strategy to select mutated OriR sequences endowed with the ability to establish recombinant plasmids at higher multiplicity per cell. The solution to this major production bottleneck was achieved by the construction of a library of psychrophilic vectors, each containing a randomly mutated version of pMtBL OriR, and its screening by fluorescence-activated cell sorting (FACS). The selected clones allowed the identification of mutated OriR sequences effective in enhancing the plasmid copy number of approximately two orders of magnitude, and the production of the recombinant green fluorescent protein was increased up to twenty times approximately. Moreover, the molecular characterization of the different mutant OriR sequences allowed us to suggest some preliminary clues on the pMtBL replication mechanism that deserve to be further investigated in the future. KEY POINTS: • Setup of an electroporation procedure for Pseudoalteromonas haloplanktis TAC125. • Two order of magnitude improvement of OriR-derived psychrophilic expression systems. • Almost twenty times enhancement in Green fluorescent protein production.

Active human full-length CDKL5 produced in the Antarctic bacterium Pseudoalteromonas haloplanktis TAC125

Background

A significant fraction of the human proteome is still inaccessible to in vitro studies since the recombinant production of several proteins failed in conventional cell factories. Eukaryotic protein kinases are difficult-to-express in heterologous hosts due to folding issues both related to their catalytic and regulatory domains. Human CDKL5 belongs to this category. It is a serine/threonine protein kinase whose mutations are involved in CDKL5 Deficiency Disorder (CDD), a severe neurodevelopmental pathology still lacking a therapeutic intervention. The lack of successful CDKL5 manufacture hampered the exploitation of the otherwise highly promising enzyme replacement therapy. As almost two-thirds of the enzyme sequence is predicted to be intrinsically disordered, the recombinant product is either subjected to a massive proteolytic attack by host-encoded proteases or tends to form aggregates. Therefore, the use of an unconventional expression system can constitute a valid alternative to solve these issues.

Results

Using a multiparametric approach we managed to optimize the transcription of the CDKL5 gene and the synthesis of the recombinant protein in the Antarctic bacterium Pseudoalteromonas haloplanktis TAC125 applying a bicistronic expression strategy, whose generalization for recombinant expression in the cold has been here confirmed with the use of a fluorescent reporter. The recombinant protein largely accumulated as a full-length product in the soluble cell lysate. We also demonstrated for the first time that full-length CDKL5 produced in Antarctic bacteria is catalytically active by using two independent assays, making feasible its recovery in native conditions from bacterial lysates as an active product, a result unmet in other bacteria so far. Finally, the setup of an in cellulo kinase assay allowed us to measure the impact of several CDD missense mutations on the kinase activity, providing new information towards a better understanding of CDD pathophysiology.

Conclusions

Collectively, our data indicate that P. haloplanktis TAC125 can be a valuable platform for both the preparation of soluble active human CDKL5 and the study of structural–functional relationships in wild type and mutant CDKL5 forms. Furthermore, this paper further confirms the more general potentialities of exploitation of Antarctic bacteria to produce “intractable” proteins, especially those containing large intrinsically disordered regions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01939-6.

Soluble Recombinant Protein Production in Pseudoalteromonas haloplanktis TAC125: The Case Study of the Full-Length Human CDKL5 Protein

The Antarctic bacterium Pseudoalteromonas haloplanktis TAC125 is an unconventional protein production host displaying a notable proficiency in the soluble production of difficult proteins, especially of human origin. Furthermore, the accumulation of recombinant products in insoluble aggregates has never been observed in this bacterium, indicating that its cellular physicochemical conditions and/or folding processes are rather different from those observed in mesophilic bacteria. The ability of this cell factory was challenged by producing a human protein, the cyclin-dependent kinase-like 5 (hCDKL5) in the bacterium cytoplasm at 0 °C. Human CDKL5 is a serine/threonine protein kinase characterized by the absence of a defined structure for the last two/third of its sequence, one of the largest intrinsically disordered regions so far observed in a human protein. This large unstructured domain makes difficult its production in most of the conventional hosts since the recombinant product accumulates as insoluble aggregates and/or is heavily proteolyzed. As the full-length hCDKL5 production is of great interest both for basic science and as protein drug for an enzyme replacement therapy, its production in the Antarctic bacterium was tested by combining the use of a regulated psychrophilic gene expression system with the use of a defined growth medium optimized for the host growth at subzero temperature. This is the first report of soluble and full-length recombinant production of hCDKL5 protein in a bacterium.

Recombinant Protein Production and Purification of Insoluble Proteins

Proteins are synthesized in heterologous systems because of the impossibility to obtain satisfactory yields from natural sources. The efficient production of soluble and functional recombinant proteins is among the main goals in the biotechnological field. In this context, it is important to point out that under stress conditions, protein folding machinery is saturated and this promotes protein misfolding and, consequently, protein aggregation. Thus, the selection of the optimal expression organism and its growth conditions to minimize the formation of insoluble protein aggregates should be done according to the protein characteristics and downstream requirements. Escherichia coli is the most popular recombinant protein expression system despite the great development achieved so far by eukaryotic expression systems. Besides, other prokaryotic expression systems, such as lactic acid bacteria and psychrophilic bacteria, are gaining interest in this field. However, it is worth mentioning that prokaryotic expression system poses, in many cases, severe restrictions for a successful heterologous protein production. Thus, eukaryotic systems such as mammalian cells, insect cells, yeast, filamentous fungus, and microalgae are an interesting alternative for the production of these difficult-to-express proteins.

Soluble Recombinant Protein Production in Pseudoalteromonas haloplanktis TAC125: The Case Study of the Full-Length Human CDKL5 Protein

The Antarctic bacterium Pseudoalteromonas haloplanktis TAC125 is an unconventional protein production host displaying a notable proficiency in the soluble production of difficult proteins, especially of human origin. Furthermore, the accumulation of recombinant products in insoluble aggregates has never been observed in this bacterium, indicating that its cellular physicochemical conditions and/or folding processes are rather different from those observed in mesophilic bacteria. The ability of this cell factory was challenged by producing a human protein, the cyclin-dependent kinase-like 5 (hCDKL5) in the bacterium cytoplasm at 0 °C. Human CDKL5 is a serine/threonine protein kinase characterized by the absence of a defined structure for the last two/third of its sequence, one of the largest intrinsically disordered regions so far observed in a human protein. This large unstructured domain makes difficult its production in most of the conventional hosts since the recombinant product accumulates as insoluble aggregates and/or is heavily proteolyzed. As the full-length hCDKL5 production is of great interest both for basic science and as protein drug for an enzyme replacement therapy, its production in the Antarctic bacterium was tested by combining the use of a regulated psychrophilic gene expression system with the use of a defined growth medium optimized for the host growth at subzero temperature. This is the first report of soluble and full-length recombinant production of hCDKL5 protein in a bacterium.

Modelling hCDKL5 Heterologous Expression in Bacteria

hCDKL5 refers to the human cyclin-dependent kinase like 5 that is primarily expressed in the brain. Mutations in its coding sequence are often causative of hCDKL5 deficiency disorder, a devastating neurodevelopmental disorder currently lacking a cure. The large-scale recombinant production of hCDKL5 is desirable to boost the translation of preclinical therapeutic approaches into the clinic. However, this is hampered by the intrinsically disordered nature of almost two-thirds of the hCDKL5 sequence, making this region more susceptible to proteolytic attack, and the observed toxicity when the enzyme is accumulated in the cytoplasm of eukaryotic host cells. The bacterium Pseudoalteromonas haloplanktis TAC125 (PhTAC125) is the only prokaryotic host in which the full-length production of hCDKL5 has been demonstrated. To date, a system-level understanding of the metabolic burden imposed by hCDKL5 production is missing, although it would be crucial for upscaling of the production process. Here, we combined experimental data on protein production and nutrients assimilation with metabolic modelling to infer the global consequences of hCDKL5 production in PhTAC125 and to identify potential overproduction targets. Our analyses showed a remarkable accuracy of the model in simulating the recombinant strain phenotype and also identified priority targets for optimised protein production.

Enhanced Recombinant Protein Production Under Special Environmental Stress

Regardless of bacteria or eukaryotic microorganism hosts, improving their ability to express heterologous proteins is always a goal worthy of elaborate study. In addition to traditional methods including intracellular synthesis process regulation and extracellular environment optimization, some special or extreme conditions can also be employed to create an enhancing effect on heterologous protein production. In this review, we summarize some extreme environmental factors used for the improvement of heterologous protein expression, including low temperature, hypoxia, microgravity and high osmolality. The applications of these strategies are elaborated with examples of well-documented studies. We also demonstrated the confirmed or hypothetical mechanisms of environment stress affecting the host behaviors. In addition, multi-omics techniques driving the stress-responsive research for construction of efficient microbial cell factories are also prospected at the end.

Improvement of Pseudoalteromonas haloplanktis TAC125 as a Cell Factory: IPTG-Inducible Plasmid Construction and Strain Engineering

Our group has used the marine bacterium Pseudoalteromonas haloplanktis TAC125 (PhTAC125) as a platform for the successful recombinant production of “difficult” proteins, including eukaryotic proteins, at low temperatures. However, there is still room for improvement both in the refinement of PhTAC125 expression plasmids and in the bacterium’s intrinsic ability to accumulate and handle heterologous products. Here, we present an integrated approach of plasmid design and strain engineering finalized to increment the recombinant expression and optimize the inducer uptake in PhTAC125. To this aim, we developed the IPTG-inducible plasmid pP79 and an engineered PhTAC125 strain called KrPL LacY⁺. This mutant was designed to express the E. coli lactose permease and to produce only a truncated version of the endogenous Lon protease through an integration-deletion strategy. In the wild-type strain, pP79 assured a significantly better production of two reporters in comparison to the most recent expression vector employed in PhTAC125. Nevertheless, the use of KrPL LacY⁺ was crucial to achieving satisfying production levels using reasonable IPTG concentrations, even at 0 °C. Both the wild-type and the mutant recombinant strains are characterized by an average graded response upon IPTG induction and they will find different future applications depending on the desired levels of expression.

Diauxie and co-utilization of carbon sources can coexist during bacterial growth in nutritionally complex environments

It is commonly thought that when multiple carbon sources are available, bacteria metabolize them either sequentially (diauxic growth) or simultaneously (co-utilization). However, this view is mainly based on analyses in relatively simple laboratory settings. Here we show that a heterotrophic marine bacterium, Pseudoalteromonas haloplanktis, can use both strategies simultaneously when multiple possible nutrients are provided in the same growth experiment. The order of nutrient uptake is partially determined by the biomass yield that can be achieved when the same compounds are provided as single carbon sources. Using transcriptomics and time-resolved intracellular ¹H-¹³C NMR, we reveal specific pathways for utilization of various amino acids. Finally, theoretical modelling indicates that this metabolic phenotype, combining diauxie and co-utilization of substrates, is compatible with a tight regulation that allows the modulation of assimilatory pathways.

It is thought that when multiple carbon sources are available, bacteria metabolize them either sequentially or simultaneously. Here, the authors show that a marine bacterium can use a mixed strategy when multiple possible nutrients are provided, and analyse the metabolic pathways involved.

New insights on Pseudoalteromonas haloplanktis TAC125 genome organization and benchmarks of genome assembly applications using next and third generation sequencing technologies

Pseudoalteromonas haloplanktis TAC125 is among the most commonly studied bacteria adapted to cold environments. Aside from its ecological relevance, P. haloplanktis has a potential use for biotechnological applications. Due to its importance, we decided to take advantage of next generation sequencing (Illumina) and third generation sequencing (PacBio and Oxford Nanopore) technologies to resequence its genome. The availability of a reference genome, obtained using whole genome shotgun sequencing, allowed us to study and compare the results obtained by the different technologies and draw useful conclusions for future de novo genome assembly projects. We found that assembly polishing using Illumina reads is needed to achieve a consensus accuracy over 99.9% when using Oxford Nanopore sequencing, but not in PacBio sequencing. However, the dependency of consensus accuracy on coverage is lower in Oxford Nanopore than in PacBio, suggesting that a cost-effective solution might be the use of low coverage Oxford Nanopore sequencing together with Illumina reads. Despite the differences in consensus accuracy, all sequencing technologies revealed the presence of a large plasmid, pMEGA, which was undiscovered until now. Among the most interesting features of pMEGA is the presence of a putative error-prone polymerase regulated through the SOS response. Aside from the characterization of the newly discovered plasmid, we confirmed the sequence of the small plasmid pMtBL and uncovered the presence of a potential partitioning system. Crucially, this study shows that the combination of next and third generation sequencing technologies give us an unprecedented opportunity to characterize our bacterial model organisms at a very detailed level.

The art of adapting to extreme environments: The model system Pseudoalteromonas

Extremophilic microbes have adapted to thrive in ecological niches characterized by harsh chemical/physical conditions such as, for example, very low/high temperature. Living organisms inhabiting these environments have developed peculiar mechanisms to cope with extreme conditions, in such a way that they mark the chemical-physical boundaries of life on Earth. Studying such mechanisms is stimulating from a basic research viewpoint and because of biotechnological applications. Pseudoalteromonas species are a group of marine gamma-proteobacteria frequently isolated from a range of extreme environments, including cold habitats and deep-sea sediments. Since deep-sea floors constitute almost 60% of the Earth's surface and cold temperatures represent the most common of the extreme conditions, the genus Pseudoalteromonas can be considered one of the most important model systems for studying microbial adaptation. Particularly, among all Pseudoalteromonas representatives, P. haloplanktis TAC125 has recently gained a central role. This bacterium was isolated from seawater sampled along the Antarctic ice-shell and is considered one of the model organisms of cold-adapted bacteria. It is capable of thriving in a wide temperature range and it has been suggested as an alternative host for the soluble overproduction of heterologous proteins, given its ability to rapidly multiply at low temperatures. In this review, we will present an overview of the recent advances in the characterization of Pseudoalteromonas strains and, more importantly, in the understanding of their evolutionary and chemical-physical strategies to face such a broad array of extreme conditions. A particular attention will be given to systems-biology approaches in the study of the above-mentioned topics, as genome-scale datasets (e.g. genomics, proteomics, phenomics) are beginning to expand for this group of organisms. In this context, a specific section dedicated to P. haloplanktis TAC125 will be presented to address the recent efforts in the elucidation of the metabolic rewiring of the organisms in its natural environment (Antarctica).

General introduction: recombinant protein production and purification of insoluble proteins

Proteins are synthesized in heterologous systems because of the impossibility to obtain satisfactory yields from natural sources. The production of soluble and functional recombinant proteins is among the main goals in the biotechnological field. In this context, it is important to point out that under stress conditions, protein folding machinery is saturated and this promotes protein misfolding and, consequently, protein aggregation. Thus, the selection of the optimal expression organism and the most appropriate growth conditions to minimize the formation of insoluble proteins should be done according to the protein characteristics and downstream requirements. Escherichia coli is the most popular recombinant protein expression system despite the great development achieved so far by eukaryotic expression systems. Besides, other prokaryotic expression systems, such as lactic acid bacteria and psychrophilic bacteria, are gaining interest in this field. However, it is worth mentioning that prokaryotic expression system poses, in many cases, severe restrictions for a successful heterologous protein production. Thus, eukaryotic systems such as mammalian cells, insect cells, yeast, filamentous fungus, and microalgae are an interesting alternative for the production of these difficult-to-express proteins.

The globins of cold-adapted Pseudoalteromonas haloplanktis TAC125: from the structure to the physiological functions

Evolution allowed Antarctic microorganisms to grow successfully under extreme conditions (low temperature and high O2 content), through a variety of structural and physiological adjustments in their genomes and development of programmed responses to strong oxidative and nitrosative stress. The availability of genomic sequences from an increasing number of cold-adapted species is providing insights to understand the molecular mechanisms underlying crucial physiological processes in polar organisms. The genome of Pseudoalteromonas haloplanktis TAC125 contains multiple genes encoding three distinct truncated globins exhibiting the 2/2 α-helical fold. One of these globins has been extensively characterised by spectroscopic analysis, kinetic measurements and computer simulation. The results indicate unique adaptive structural properties that enhance the overall flexibility of the protein, so that the structure appears to be resistant to pressure-induced stress. Recent results on a genomic mutant strain highlight the involvement of the cold-adapted globin in the protection against the stress induced by high O2 concentration. Moreover, the protein was shown to catalyse peroxynitrite isomerisation in vitro. In this review, we first summarise how cold temperatures affect the physiology of microorganisms and focus on the molecular mechanisms of cold adaptation revealed by recent biochemical and genetic studies. Next, since only in a very few cases the physiological role of truncated globins has been demonstrated, we also discuss the structural and functional features of the cold-adapted globin in an attempt to put into perspective what has been learnt about these proteins and their potential role in the biology of cold-adapted microorganisms.

A dynamic method for the investigation of induced state metabolic capacities as a function of temperature

Background

Science-based recombinant bioprocess designs as well as the design of statistical experimental plans for process optimization (Design of Experiments, DoE) demand information on physiological bioprocess boundaries, such as the onset of acetate production, adaptation times, mixed feed metabolic capabilities or induced state maximum metabolic rates as at the desired cultivation temperature. Dynamic methods provide experimental alternatives to determine this information in a fast and efficient way. Information on maximum metabolic capabilities as a function of temperature is needed in case a reduced cultivation temperature is desirable (e.g. to avoid inclusion body formation) and an appropriate feeding profile is to be designed.

Results

Here, we present a novel dynamic method for the determination of the specific growth rate as a function of temperature for induced recombinant bacterial bioprocesses. The method is based on the control of the residual substrate concentration at non-limiting conditions with dynamic changes in cultivation temperature. The presented method was automated in respect to information extraction and closed loop control by means of in-line Fourier Transformation Infrared Spectroscopy (FTIR) residual substrate measurements and on-line first principle rate-based soft-sensors. Maximum induced state metabolic capabilities as a function of temperature were successfully extracted for a recombinant E. coli C41 fed-batch bioprocess without the need for sampling in a time frame of 20 hours.

Conclusions

The presented method was concluded to allow the fast and automated extraction of maximum metabolic capabilities (specific growth rate) as a function of temperature. This complements the dynamic toolset necessary for science-based recombinant bacterial bioprocess design and DoE design.

Unconventional microbial systems for the cost-efficient production of high-quality protein therapeutics

Both conventional and innovative biomedical approaches require cost-effective protein drugs with high therapeutic potency, improved bioavailability, biocompatibility, stability and pharmacokinetics. The growing longevity of the human population, the increasing incidence and prevalence of age-related diseases and the better comprehension of genetic-linked disorders prompt to develop natural and engineered drugs addressed to fulfill emerging therapeutic demands. Conventional microbial systems have been for long time exploited to produce biotherapeutics, competing with animal cells due to easier operation and lower process costs. However, both biological platforms exhibit important drawbacks (mainly associated to intracellular retention of the product, lack of post-translational modifications and conformational stresses), that cannot be overcome through further strain optimization merely due to physiological constraints. The metabolic diversity among microorganisms offers a spectrum of unconventional hosts, that, being able to bypass some of these weaknesses, are under progressive incorporation into production pipelines. In this review we describe the main biological traits and potentials of emerging bacterial, yeast, fungal and microalgae systems, by comparing selected leading species with well established conventional organisms with a long run in protein drug production.

Fed-batch process for the psychrotolerant marine bacterium Pseudoalteromonas haloplanktis

Background

Pseudoalteromonas haloplanktis is a cold-adapted γ-proteobacterium isolated from Antarctic sea ice. It is characterized by remarkably high growth rates at low temperatures. P. haloplanktis is one of the model organisms of cold-adapted bacteria and has been suggested as an alternative host for the soluble overproduction of heterologous proteins which tend to form inclusion bodies in established expression hosts. Despite the progress in establishing P. haloplanktis as an alternative expression host the cell densities obtained with this organism, which is unable to use glucose as a carbon source, are still low. Here we present the first fed-batch cultivation strategy for this auspicious alternative expression host.

Results

The key for the fed-batch cultivation of P. haloplanktis was the replacement of peptone by casamino acids, which have a much higher solubility and allow a better growth control. In contrast to the peptone medium, on which P. haloplanktis showed different growth phases, on a casamino acids-containing, phosphate-buffered medium P. haloplanktis grew exponentially with a constant growth rate until the stationary phase. A fed-batch process was established by feeding of casamino acids with a constant rate resulting in a cell dry weight of about 11 g l^-1 (OD₅₄₀ = 28) which is a twofold increase of the highest densities which have been obtained with P. haloplanktis so far and an eightfold increase of the density obtained in standard shake flask cultures.

The cell density was limited in the fed-batch cultivation by the relatively low solubility of casamino acids (about 100 g l^-1), which was proven by pulse addition of casamino acid powder which increased the cell density to about 20 g l^-1 (OD₅₄₀ = 55).

Conclusion

The growth of P. haloplanktis to higher cell densities on complex medium is possible. A first fed-batch fermentation strategy could be established which is feasible to be used in lab-scale or for industrial purposes. The substrate concentration of the feeding solution was found to influence the maximal biomass yield considerably. The bottleneck for growing P. haloplanktis to high cell densities still remains the availability of a highly concentrated substrate and the reduction of the substrate complexity. However, our results indicate glutamic acid as a major carbon source, which provides a good basis for further improvement of the fed-batch process.

Molecular characterization of a cryptic plasmid from the psychrotrophic antarctic bacterium Pseudoalteromonas sp. 643A

We report the identification and nucleotide sequence analysis of pKW1, a plasmid of the psychrotrophic bacterium Pseudoalteromonas sp. 643A isolated from the stomach of Antarctic krill Euphasia superba. pKW1 consists of 4583 bp, has a G+C content of 43% and seven putative open reading frames (ORFs). The deduced amino acid sequence from ORF-1 shared significant similarity with the plasmid replicase protein of Psychrobacter cryohalolentis, strain K5. The DNA region immediately downstream of the ORF-1 showed some homology with the Rep-binding sequence of the theta-replicating ColE2-type plasmids. The ORF-3 amino acid sequence revealed amino acid sequence homology with the mobilization protein of Psychrobacter sp. PRwf-1 and Moraxella catarrhalis, with identities of 28% and 25%, respectively. The ORF-4 showed 46% amino acid sequence homology with the putative relaxase/mobilization nuclease MobA of Hafnia alvei and 44% homology with the putative mobilization protein A of Pasterulla multocida. The copy number of pKW1 in Pseudoalteromonas sp. 643A was estimated of 15 copies per chromosome.

Development of an improved Pseudoalteromonas haloplanktis TAC125 strain for recombinant protein secretion at low temperature

BACKGROUND

In a previous paper, we reported the accomplishment of a cold gene-expression system for the recombinant secretion of heterologous proteins in Pseudoalteromonas haloplanktis TAC125. This system makes use of the psychrophilic alpha-amylase from P. haloplanktis TAB23 as secretion carrier, and allows an effective extra-cellular addressing of recombinant proteins. However, Pseudoalteromonales are reported to secrete a wide range of extra-cellular proteases. This feature works against the efficiency of the cold-adapted secretion system, because of the proteolytic degradation of recombinant products. The aim of this study is the construction of a P. haloplanktis TAC125 mutant strain with reduced extra-cellular proteolytic activity.

RESULTS

P. haloplanktis TAC125 culture medium resulted to contain multiple and heterogeneous proteases. Since the annotation of the Antarctic bacterium genome highlighted the presence of only one canonical secretion machinery, namely the Type II secretion pathway (T2SS), we have inactivated this secretion system by a gene insertion strategy. A mutant strain of P. haloplanktis TAC125 in which the gspE gene was knocked-out, actually displayed a remarkable reduction of the extra-cellular protease secretion. Quite interestingly this strain still retained the ability to secrete the psychrophilic amylase as efficiently as the wild type. Moreover, the decrease in extra-cellular proteolytic activity resulted in a substantial improvement in the stability of the secreted amylase-beta-lactamase chimera.

CONCLUSION

Here we report a cell engineering approach to the construction of a P. haloplanktis TAC125 strain with reduced extra-cellular protease activity. The improved strain is able to secrete the psychrophilic alpha-amylase (the carrier of our recombinant secretion system), while it displays a significant reduction of protease content in the culture medium. These features make the gspE mutant an improved host with a remarkable biotechnological potential in recombinant protein secretion at low temperature. Moreover this work demonstrates that P. haloplanktis TAC125 is a versatile psychrophilic host for recombinant protein production since it can be easily improved by a directed engineering approach. To the best of our knowledge, this is the first described example of a strain improvement strategy applied to an Antarctic bacterium.

Genetics for Pseudoalteromonas provides tools to manipulate marine bacterial virus PM2

The genetic manipulation of marine double-stranded DNA (dsDNA) bacteriophage PM2 (Corticoviridae) has been limited so far. The isolation of an autonomously replicating DNA element of Pseudoalteromonas haloplanktis TAC125 and construction of a shuttle vector replicating in both Escherichia coli and Pseudoalteromonas enabled us to design a set of conjugative shuttle plasmids encoding tRNA suppressors for amber mutations. Using a host strain carrying a suppressor plasmid allows the introduction and analysis of nonsense mutations in PM2. Here, we describe the isolation and characterization of a suppressor-sensitive PM2 sus2 mutant deficient in the structural protein P10. To infect and replicate, PM2 delivers its 10-kbp genome across the cell envelopes of two gram-negative Pseudoalteromonas species. The events leading to the internalization of the circular supercoiled dsDNA are puzzling. In a poorly understood process that follows receptor recognition, the virion capsid disassembles and the internal membrane fuses with the host outer membrane. While beginning to unravel the mechanism of this process, we found that protein P10 plays an essential role in the host cell penetration.

A novel genetic system for recombinant protein secretion in the Antarctic Pseudoalteromonas haloplanktis TAC125

BACKGROUND

The final aim of recombinant protein production is both to have a high specific production rate and a high product quality. It was already shown that using cold-adapted bacteria as host vectors, some "intractable" proteins can be efficiently produced at temperature as low as 4 degrees C.

RESULTS

A novel genetic system for the production and secretion of recombinant proteins in the Antarctic Gram-negative bacterium Pseudoalteromonas haloplanktis TAC125 was set up. This system aims at combining the low temperature recombinant product production with the advantages of extra-cellular protein targeting. The psychrophilic alpha-amylase from Pseudoalteromonas haloplanktis TAB23 was used as secretion carrier. Three chimerical proteins were produced by fusing intra-cellular proteins to C-terminus of the psychrophilic alpha-amylase and their secretion was analysed. Data reported in this paper demonstrate that all tested chimeras were translocated with a secretion yield always higher than 80%.

CONCLUSION

Data presented here demonstrate that the "cold" gene-expression system is efficient since the secretion yield of tested chimeras is always above 80%. These secretion performances place the alpha-amylase derived secretion system amongst the best heterologous secretion systems in Gram-negative bacteria reported so far. As for the quality of the secreted passenger proteins, data presented suggest that the system also allows the correct disulphide bond formation of chimera components, secreting a fully active passenger.