Structural Insights on PHA Binding Protein PhaP from Aeromonas hydrophila

Efficient bioproduction of poly(3-hydroxypropionate) homopolymer using engineered Escherichia coli strains

This study focuses on the development of a scalable method for producing poly(3-hydroxypropionate), a homopolymer with significant physico-mechanical properties, through the use of metabolically-engineered Escherichia coli K12 (MG1655) and externally supplied 3-hydroxypropionate. The polymer synthesis pathway was established and optimized through synthetic biology techniques, including the effects of overexpressing phasin and cell division proteins. The optimized strain achieved unprecedented production titers of 9.5 g/L in flask cultures and 80 g/L in fed-batch bioreactors within 45 h. The analysis of poly(3-hydroxypropionate) polymer properties revealed it possesses excellent elasticity (Young's modulus < 6 MPa) and tensile strength (∼80 MPa), positioning it within the category of elastomers or flexible plastics. These findings suggest a viable path for the sustainable, large-scale production of the poly(3-hydroxypropionate) biopolymer.

Computational Analysis of the Tripartite Interaction of Phasins (PhaP4 and 5)-Sigma Factor (σ24)-DNA of Azospirillum brasilense Sp7

Azospirillum brasilense Sp7 produces PHB, which is covered by granule-associated proteins (GAPs). Phasins are the main GAPs. Previous studies have shown phasins can regulate PHB synthesis. When A. brasilense grows under stress conditions, it uses sigma factors to transcribe genes for survival. One of these factors is the σ24 factor. This study determined the possible interaction between phasins and the σ24 factor or phasin-σ24 factor complex and DNA. Three-dimensional structures of phasins and σ24 factor structures were predicted using the I-TASSER and SWISS-Model servers, respectively. Subsequently, a molecular docking between phasins and the σ24 factor was performed using the ClusPro 2.0 server, followed by molecular docking between protein complexes and DNA using the HDOCK server. Evaluation of the types of ligand-receptor interactions was performed using the BIOVIA Discovery Visualizer for three-dimensional diagrams, as well as the LigPlot server to obtain bi-dimensional diagrams. The results showed the phasins (Pha4Abs7 or Pha5Abs7)-σ24 factor complex was bound near the -35 box of the promoter region of the phaC gene. However, in the individual interaction of PhaP5Abs7 and the σ24 factor, with DNA, both proteins were bound to the -35 box. This did not occur with PhaP4Abs7, which was bound to the -10 box. This change could affect the transcription level of the phaC gene and possibly affect PHB synthesis.

Pleiotropic Effects of PhaR Regulator in Bradyrhizobium diazoefficiens Microaerobic Metabolism

Bradyrhizobium diazoefficiens can live inside soybean root nodules and in free-living conditions. In both states, when oxygen levels decrease, cells adjust their protein pools by gene transcription modulation. PhaR is a transcription factor involved in polyhydroxyalkanoate (PHA) metabolism but also plays a role in the microaerobic network of this bacterium. To deeply uncover the function of PhaR, we applied a multipronged approach, including the expression profile of a phaR mutant at the transcriptional and protein levels under microaerobic conditions, and the identification of direct targets and of proteins associated with PHA granules. Our results confirmed a pleiotropic function of PhaR, affecting several phenotypes, in addition to PHA cycle control. These include growth deficiency, regulation of carbon and nitrogen allocation, and bacterial motility. Interestingly, PhaR may also modulate the microoxic-responsive regulatory network by activating the expression of fixK2 and repressing nifA, both encoding two transcription factors relevant for microaerobic regulation. At the molecular level, two PhaR-binding motifs were predicted and direct control mediated by PhaR determined by protein-interaction assays revealed seven new direct targets for PhaR. Finally, among the proteins associated with PHA granules, we found PhaR, phasins, and other proteins, confirming a dual function of PhaR in microoxia.

Nature-inspired material binding peptides with versatile polyester affinities and binding strengths

Biodegradable polyesters, such as polyhydroxyalkanoates (PHAs), are having a tremendous impact on biomedicine. However, these polymers lack functional moieties to impart functions like targeted delivery of molecules. Inspired by native GAPs, such as phasins and their polymer-binding and surfactant properties, we generated small material binding peptides (MBPs) for polyester surface functionalization using a rational approach based on amphiphilicity. Here, two peptides of 48 amino acids derived from phasins PhaF and PhaI from Pseudomonas putida, MinP and the novel-designed MinI, were assessed for their binding towards two types of PHAs, PHB and PHOH. In vivo, fluorescence studies revealed selective binding towards PHOH, whilst in vitro binding experiments using the Langmuir-Blodgett technique coupled to ellipsometry showed KD in the range of nM for all polymers and MBPs. Marked morphological changes of the polymer surface upon peptide adsorption were shown by BAM and AFM for PHOH. Moreover, both MBPs were successfully used to immobilize cargo proteins on the polymer surfaces. Altogether, this work shows that by redesigning the amphiphilicity of phasins, a high affinity but lower specificity to polyesters can be achieved in vitro. Furthermore, the MBPs demonstrated binding to PET, showing potential to bind cargo molecules also to synthetic polyesters.

Enzybiotic-mediated antimicrobial functionalization of polyhydroxyalkanoates

Polymeric nanoparticles (NPs) present some ideal properties as biomedical nanocarriers for targeted drug delivery such as enhanced translocation through body barriers. Biopolymers, such as polyhydroxyalkanoates (PHAs) are gaining attention as nanocarrier biomaterials due to their inherent biocompatibility, biodegradability, and ability to be vehiculized through hydrophobic media, such as the lung surfactant (LS). Upon colonization of the lung alveoli, below the LS layer, Streptococcus pneumoniae, causes community-acquired pneumonia, a severe respiratory condition. In this work, we convert PHA NPs into an antimicrobial material by the immobilization of an enzybiotic, an antimicrobial enzyme, via a minimal PHA affinity tag. We first produced the fusion protein M711, comprising the minimized PHA affinity tag, MinP, and the enzybiotic Cpl-711, which specifically targets S. pneumoniae. Then, a PHA nanoparticulate suspension with adequate physicochemical properties for pulmonary delivery was formulated, and NPs were decorated with M711. Finally, we assessed the antipneumococcal activity of the nanosystem against planktonic and biofilm forms of S. pneumoniae. The resulting system displayed sustained antimicrobial activity against both, free and sessile cells, confirming that tag-mediated immobilization of enzybiotics on PHAs is a promising platform for bioactive antimicrobial functionalization.

Genome-centric polyhydroxyalkanoate reconciliation reveals nutrient enriched growth dependent biosynthesis in Bacillus cereus IBA1

Microbiological polyhydroxyalkanoates (PHAs) are rooted as the most promising bio-replacements of synthetic polymers. Inherent properties of these PHAs further expand their applicability in numerous industrial, environmental, and clinical sectors. To propel these, a new environmental, endotoxin free gram-positive bacterium i.e., Bacillus cereus IBA1 was identified to harbor advantageous PHA producer characteristics through high-throughput omics mining approaches. Unlike traditional fermentations, nutrient enriched strategy was used to enhance PHA granular concentrations by ∼2.3 folds to 2.78 ± 0.19 g/L. Additionally, this study is the first to confirm an underlying growth dependent PHA biogenesis through exploring PHA granule associated operons which harbour constitutively expressing PHA synthase (phaC) coupled with differentially expressing PHA synthase subunit (phaR) and regulatory protein (phaP, phaQ) amid different growth phases. Moreover, the feasibility of this promising microbial phenomenon could propel next-generation biopolymers, and increase industrial applicability of PHAs, thereby significantly contributing to the sustainable development.

Complete genome sequence of Aquitalea pelogenes USM4 (JCM19919), a polyhydroxyalkanoate producer

Polyhydroxyalkanoate (PHA) is a type of biopolymer produced by most bacteria and archaea, resembling thermoplastic with biodegradability and biocompatibility features. Here, we report the complete genome of a PHA producer, Aquitalea sp. USM4, isolated from Perak, Malaysia. This bacterium possessed a 4.2 Mb circular chromosome and a 54,370 bp plasmid. A total of 4067 predicted protein-coding sequences, 87 tRNA genes, and 25 rRNA operons were identified using PGAP. Based on ANI and dDDH analysis, the Aquitalea sp. USM4 is highly similar to Aquitalea pelogenes. We also identified genes, including acetyl-CoA (phaA), acetoacetyl-CoA (phaB), PHA synthase (phaC), enoyl-CoA hydratase (phaJ), and phasin (phaP), which play an important role in PHA production in Aquitalea sp. USM4. The heterologous expression of phaC1 from Aquitalea sp. USM4 in Cupriavidus necator PHB^-4 was able to incorporate six different types of PHA monomers, which are 3-hydroxybutyrate (3HB), 3-hydroxyvalerate (3HV), 4-hydroxybutyrate (4HB), 5-hydroxyvalerate (5HV), 3-hydroxyhexanoate (3HHx) and isocaproic acid (3H4MV) with suitable precursor substrates. This is the first complete genome sequence of the genus Aquitalea among the 22 genome sequences from 4 Aquitalea species listed in the GOLD database, which provides an insight into its genome evolution and molecular machinery responsible for PHA biosynthesis.

Decoding the mechanism of vascular morphogenesis to explore future prospects in targeted tumor therapy

The growth and formation of blood vessels is an undeniably fundamental biological process crucial to controlling overall development of an organism. This phenomenon consists of two separate processes, commencing with vasculogenesis, which refers to the process of blood vessel formation strictly in embryonic stages, via de novo endothelial cell synthesis. Angiogenesis continues the formation of the vascular network via sprouting and splitting. Tumor growth is dependent on the growth and supply of blood vessels around the tumor mass. Extracellular matrix (ECM) molecules can promote angiogenesis by establishing a vascular network and sequestering pro-angiogenic growth factors. Although the methods by which tumor-associated fibroblasts (which differ in phenotype from normal fibroblasts) influence angiogenesis are unknown, they are thought to be a major source of growth factors and cytokines that attract endothelial cells. Chemokines and growth factors (sourced from macrophages and neutrophils) are also regulators of angiogenesis. When considered as a whole, the tumor microenvironment is a heterogenous and dynamic mass of tissue, composed of a plethora of cell types and an ECM that can fundamentally control the pathological angiogenic switch. Angiogenesis is involved in numerous diseases, and understanding the various mechanisms surrounding this phenomenon is key to finding cures.

Functional Analysis of Hypothetical Proteins of Vibrio parahaemolyticus Reveals the Presence of Virulence Factors and Growth-Related Enzymes With Therapeutic Potential

Vibrio parahaemolyticus, an aquatic pathogen, is a major concern in the shrimp aquaculture industry. Several strains of this pathogen are responsible for causing acute hepatopancreatic necrosis disease as well as other serious illness, both of which result in severe economic losses. The genome sequence of two pathogenic strains of V. parahaemolyticus, MSR16 and MSR17, isolated from Bangladesh, have been reported to gain a better understanding of their diversity and virulence. However, the prevalence of hypothetical proteins (HPs) makes it challenging to obtain a comprehensive understanding of the pathogenesis of V. parahaemolyticus. The aim of the present study is to provide a functional annotation of the HPs to elucidate their role in pathogenesis employing several in silico tools. The exploration of protein domains and families, similarity searches against proteins with known function, gene ontology enrichment, along with protein-protein interaction analysis of the HPs led to the functional assignment with a high level of confidence for 656 proteins out of a pool of 2631 proteins. The in silico approach used in this study was important for accurately assigning function to HPs and inferring interactions with proteins with previously described functions. The HPs with function predicted were categorized into various groups such as enzymes involved in small-compound biosynthesis pathway, iron binding proteins, antibiotics resistance proteins, and other proteins. Several proteins with potential druggability were identified among them. In addition, the HPs were investigated in search of virulent factors, which led to the identification of proteins that have the potential to be exploited as vaccine candidate. The findings of the study will be effective in gaining a better understanding of the molecular mechanisms of bacterial pathogenesis. They may also provide an insight into the process of evaluating promising targets for the development of drugs and vaccines against V. parahaemolyticus.

The protective role of PHB and its degradation products against stress situations in bacteria

Many bacteria produce storage biopolymers that are mobilized under conditions of metabolic adaptation, for example, low nutrient availability and cellular stress. Polyhydroxyalkanoates are often found as carbon storage in Bacteria or Archaea, and of these polyhydroxybutyrate (PHB) is the most frequently occurring PHA type. Bacteria usually produce PHB upon availability of a carbon source and limitation of another essential nutrient. Therefore, it is widely believed that the function of PHB is to serve as a mobilizable carbon repository when bacteria face carbon limitation, supporting their survival. However, recent findings indicate that bacteria switch from PHB synthesis to mobilization under stress conditions such as thermal and oxidative shock. The mobilization products, 3-hydroxybutyrate and its oligomers, show a protective effect against protein aggregation and cellular damage caused by reactive oxygen species and heat shock. Thus, bacteria should have an environmental monitoring mechanism directly connected to the regulation of the PHB metabolism. Here, we review the current knowledge on PHB physiology together with a summary of recent findings on novel functions of PHB in stress resistance. Potential applications of these new functions are also presented.

Bio-specific immobilization of enzymes on electrospun PHB nanofibers

Enzyme immobilization provides substantial advantages in terms of improving the efficiency of enzymatic process as well as enhancing the reusability of enzymes. Phasins (PhaPs) are naturally occurring polyhydroxyalkanoate (PHA)-binding proteins, and thus can potentially be used as a fusion partner for oriented immobilization of enzymes onto PHA supports. However, presently available granular PHA supports have low surface-area-to-volume ratio and limited configurational flexibility of enzymatic reactions. In this study, we explored the use of electrospun polyhydroxybutyrate (PHB) nanofibers as an alternative support for high density immobilization of a PhaP-fused lipase. As envisioned, the electrospun PHB nanofibers could anchor 120-fold more enzyme than PHB granules of the same weight. Furthermore, the enzymes immobilized onto the PHB nanofibers exhibited markedly higher stability and activity compared to when immobilized on conventional immobilization supports. Our approach combines the advantageous features of nanofibrous material and specificity of biomolecular interaction for the efficient use of enzymes, which can be widely adopted in the development of various enzymatic processes.

Dissecting the Polyhydroxyalkanoate-Binding Domain of the PhaF Phasin: Rational Design of a Minimized Affinity Tag

Phasin PhaF from Pseudomonas putida consists of a modular protein whose N-terminal domain (BioF) has been demonstrated to be responsible for binding to the polyhydroxyalkanoate (PHA) granule. BioF has been exploited for biotechnological purposes as an affinity tag in the functionalization of PHA beads with fusion proteins both in vivo and in vitro The structural model of this domain suggests an amphipathic α-helical conformation with the hydrophobic residues facing the PHA granule. In this work, we analyzed the mean hydrophobicity and the hydrophobic moment of the native BioF tag to rationally design shorter versions that maintain affinity for the granule. Hybrid proteins containing the green fluorescent protein (GFP) fused to the BioF derivatives were studied for in vivo localization on PHA, stability on the surface of the PHA granule against pH, temperature, and ionic strength, and their possible influence on PHA synthesis. Based on the results obtained, a minimized BioF tag for PHA functionalization has been proposed (MinP) that retains similar binding properties but possesses an attractive biotechnological potential derived from its reduced size. The MinP tag was further validated by analyzing the functionality and stability of the fusion proteins MinP-β-galactosidase and MinP-CueO from Escherichia coliIMPORTANCE Polyhydroxyalkanoates (PHAs) are biocompatible, nontoxic, and biodegradable biopolymers with exceptional applications in the industrial and medical fields. The complex structure of the PHA granule can be exploited as a toolbox to display molecules of interest on their surface. Phasins, the most abundant group of proteins on the granule, have been employed as anchoring tags to obtain functionalized PHA beads for high-affinity bioseparation, enzyme immobilization, diagnostics, or cell targeting. Here, a shorter module based on the previously designed BioF tag has been demonstrated to maintain the affinity for the PHA granule, with higher stability and similar functionalization efficiency. The use of a 67% shorter peptide, which maintains the binding properties of the entire protein, constitutes an advantage for the immobilization of recombinant proteins on the PHA surface both in vitro and in vivo.

Novel unexpected functions of PHA granules

Polyhydroxyalkanoates (PHA), polyesters accumulated by numerous prokaryotes in the form of intracellular granules, have been for decades considered being predominantly storage molecules. However, numerous recent discoveries revealed and emphasized their complex biological role for microbial cells. Most of all, it was repeatedly reported and confirmed that the presence of PHA granules in prokaryotic cells enhances stress resistance and robustness of microbes against various environmental stress factors such as high or low temperature, freezing, oxidative, and osmotic pressure. It seems that protective mechanisms of PHA granules are associated with their extraordinary architecture and biophysical properties as well as with the complex and deeply interconnected nature of PHA metabolism. Therefore, this review aims at describing novel and unexpected properties of PHA granules with respect to their contribution to stress tolerance of various prokaryotes including common mesophilic heterotrophic bacteria, but also extremophiles or photo-autotrophic cyanobacteria. KEY POINTS: • PHA granules present in bacterial cells reveal unique properties and functions. • PHA enhances stress robustness of bacterial cells.

Bioengineered Polyhydroxyalkanoates as Immobilized Enzyme Scaffolds for Industrial Applications

Enzymes function as biocatalysts and are extensively exploited in industrial applications. Immobilization of enzymes using support materials has been shown to improve enzyme properties, including stability and functionality in extreme conditions and recyclability in biocatalytic processing. This review focuses on the recent advances utilizing the design space of in vivo self-assembled polyhydroxyalkanoate (PHA) particles as biocatalyst immobilization scaffolds. Self-assembly of biologically active enzyme-coated PHA particles is a one-step in vivo production process, which avoids the costly and laborious in vitro chemical cross-linking of purified enzymes to separately produced support materials. The homogeneous orientation of enzymes densely coating PHA particles enhances the accessibility of catalytic sites, improving enzyme function. The PHA particle technology has been developed into a remarkable scaffolding platform for the design of cost-effective designer biocatalysts amenable toward robust industrial bioprocessing. In this review, the PHA particle technology will be compared to other biological supramolecular assembly-based technologies suitable for in vivo enzyme immobilization. Recent progress in the fabrication of biological particulate scaffolds using enzymes of industrial interest will be summarized. Additionally, we outline innovative approaches to overcome limitations of in vivo assembled PHA particles to enable fine-tuned immobilization of multiple enzymes to enhance performance in multi-step cascade reactions, such as those used in continuous flow bioprocessing.

Designer bioemulsifiers based on combinations of different polysaccharides with the novel emulsifying esterase AXE from Bacillus subtilis CICC 20034

Background

Bioemulsifiers are surface-active compounds, which exhibit advantages including low toxicity, higher biodegradability and biocompatibility over synthetic chemical surfactants. Despite their potential benefits, some obstacles impede the practical applications of bioemulsifiers, including low yields and high purification costs. Here, we aimed to exploit a novel protein bioemulsifier with efficient emulsifying activity and low-production cost, as well as proposed a design-bioemulsifier system that meets different requirements of industrial emulsification in the most economical way.

Results

The esterase AXE was first reported for its efficient emulsifying activity and had been studied for possible application as a protein bioemulsifier. AXE showed an excellent emulsification effect with different hydrophobic substrates, especially short-chain aliphatic and benzene derivatives, as well as excellent stability under extreme conditions such as high temperature (85 °C) and acidic conditions. AXE also exhibited good stability over a range of NaCl, MgSO₄, and CaCl₂ concentrations from 0 to 1000 mM, and the emulsifying activity even showed a slight increase at salt concentrations over 500 mM. A design-bioemulsifier system was proposed that uses AXE in combination with a variety of polysaccharides to form efficient bioemulsifier, which enhanced the emulsifying activity and further lowered the concentration of AXE needed in the complex.

Conclusions

AXE showed a great application potential as a novel bioemulsifier with excellent emulsifying ability. The AXE-based-designer bioemulsifier could be obtained in the most economical way and open broad new fields for low-cost, environmentally friendly bioemulsifiers.

Phasin PhaP1 is involved in polyhydroxybutyrate granules morphology and in controlling early biopolymer accumulation in Azospirillum brasilense Sp7

Phasins are amphiphilic proteins involved in the regulation of the number and size of polyhydroxybutyrate (PHB) granules. The plant growth promoting bacterium Azospirillum brasilense Sp7 accumulates high quantities of bioplastic PHB as carbon and energy source. By analyzing the genome, we identified six genes that code for proteins with a Phasin_2 domain. To understand the role of A. brasilense Sp7 PhaP1 (PhaP1_Abs) on PHB synthesis, the phaP1 gene (AMK58_RS17065) was deleted. The morphology of the PHB granules was analyzed by transmission electron microscopy (TEM) and the PHB produced was quantified under three different C:N ratios in cultures subjected to null or low-oxygen transfer. The results showed that PhaP1_Abs is involved in PHB granules morphology and in controlling early biopolymer accumulation. Using RT-PCR it was found that phasin genes, except phaP4, are transcribed in accordance with the C:N ratio used for the growth of A. brasilense. phaP1, phaP2 and phaP3 genes were able to respond to the growth conditions tested. This study reports the first analysis of a phasin protein in A. brasilense Sp7.

Role of leucine zipper-like motifs in the oligomerization of Pseudomonas putida phasins

BACKGROUND

Phasins are low molecular mass proteins that accumulate strongly in bacterial cells in response to the intracellular storage of polyhydroxyalkanoates (PHA). Although lacking catalytic activity, phasins are the major components of the surface of the PHA granules and could be potentially involved in the formation of a network-like protein layer surrounding the polyester inclusions. Structural models revealed phasins to possess coiled-coil regions that might be important in the establishment of protein-protein interactions. However, there is not experimental evidence of a coiled-coil mediated oligomerization in these proteins.

METHODS

Structure prediction analyses were used to characterize the coiled-coil motifs of phasins PhaF and PhaI -produced by the model bacterium Pseudomonas putida KT2440-. Their oligomerization was evaluated by biolayer interferometry and the in vivo two-hybrid (BACTH) system. The interaction ability of a series of coiled-coil mutated derivatives was also measured.

RESULTS

The formation of PhaF and PhaI complexes was detected. A predicted short leucine zipper-like coiled-coil (ZIP), containing "ideal" residues located within the hydrophobic core, was shown responsible for the oligomers stability. The substitution of key residues (leucines or valines) in PhaI ZIP (ZIPI) for alanine reduced by four fold the oligomerization efficiency.

CONCLUSIONS

These results indicate that coiled-coil motifs are essential for phasin interactions. Correct oligomerization requires the formation of a stable hydrophobic interface between both phasins.

GENERAL SIGNIFICANCE

Our findings elucidate the oligomerization motif of PhaF and PhaI. This motif is present in most phasins from PHA-accumulating bacteria and offers a potentially important target for modulating the PHA granules stability.

Structural Insights into Polyhydroxyalkanoates Biosynthesis

Polyhydroxyalkanoates (PHAs) are diverse biopolyesters produced by numerous microorganisms and have attracted much attention as a substitute for petroleum-based polymers. Despite several decades of study, the detailed molecular mechanisms of PHA biosynthesis have remained unknown due to the lack of structural information on the key PHA biosynthetic enzyme PHA synthase. The recently determined crystal structure of PHA synthase, together with the structures of acetyl-coenzyme A (CoA) acetyltransferase and reductase, have changed this situation. Structural and biochemical studies provided important clues for the molecular mechanisms of each enzyme as well as the overall mechanism of PHA biosynthesis from acetyl-CoA. This new information and knowledge is expected to facilitate production of designed novel PHAs and also enhanced production of PHAs.

Reprogramming Halomonas for industrial production of chemicals

Halomonas spp. are able to grow under a high salt concentration at alkali pH, they are able to resist contamination by other microbes. Development of Halomonas spp. as platform production strains for the next-generation industrial biotechnology (NGIB) is intensively studied. Among Halomonas spp., Halomonas bluephagenesis is the best studied one with available engineering tools and methods to reprogram it for production of various polyhydroxyalkanoates, proteins, and chemicals. Due to its contamination resistance, H. bluephagenesis can be grown under open and continuous processes not just in the labs but also in at least 1000 L fermentor scale. It is expected that NGIB based on Halomonas spp. be able to engineer for production of increasing number of products in a competitive manner.

Halomonas and Pathway Engineering for Bioplastics Production

Traditional microbial chassis, including Escherichia coli, Bacillus subtilis, Ralstonia eutropha, and Pseudomonas putida, are grown under neutral pH and mild osmotic pressure for production of chemicals and materials. They tend to be contaminated easily by many microorganisms. To address this issue, next-generation industrial biotechnology employing halophilic Halomonas spp. has been developed for production of bioplastics polyhydroxyalkanoates (PHAs) and other chemicals. Halomonas spp. that can be grown contamination free under open and unsterile condition at alkali pH and high NaCl have been engineered to produce several PHA polymers in elongated or enlarged cells. New pathways can also be constructed both in plasmids and on chromosomes for Halomonas spp. Synthetic biology approaches and parts have been developed for Halomonas spp., allowing better control of their growth and product formation as well as morphology adjustment. Halomonas spp. and their synthetic biology will play an increasingly important role for industrial production of large volume chemicals.

Development and optimization of a tumor targeting system based on microbial synthesized PHA biopolymers and PhaP mediated functional modification

Polyhydroxyalkanoate (PHA) is a class of microbial synthesized biodegradable and biocompatible aliphatic polymer which has been developed into nanoparticles (NPs) for sustained release of hydrophobic compounds. Taking advantage of the natural PHA binding protein PhaP which could be steadily adsorbed onto PHA NPs through hydrophobic interaction, a tumor targeting system was developed in this study by presenting an epidermal growth factor receptor (EGFR)-targeting peptide (ETP) on the surface of PHA NPs, via PhaP mediated adsorption. To reveal the effects of residual emulsifiers on PhaP mediated ETP modification and optimize the tumor targeting capacity of the system, a novel emulsifier-free PHA NPs (EF-NPs) was fabricated together with other two kinds of conventional emulsifier-required PHA NPs (PVA-NPs and P68-NPs, which were prepared with poly(vinyl alcohol) (PVA) and Pluronic F68 as emulsifiers, respectively). By analyzing the surface hydrophobicity, the amount of adsorbed fusion protein, and the cellular uptake of all kinds of PHA NPs, our results demonstrated that EF-NPs with stronger surface hydrophobicity were the most proper formulation for further PhaP mediated ETP functionalization. The residual PVA and Pluronic F68 affected the modification efficiency and secondary structure of ETP-PhaP fusion protein, and finally obstructed the targeting effect of ETP-PhaP modified PVA-NPs and P68-NPs to EGFR over-expressed tumor cells. The animal experiment further confirmed the effectiveness and feasibility of in vivo application of ETP-PhaP functionalized EF-NPs, indicating that it could be served as a promising tumor targeting system with satisfactory EGFR targeting ability. This PhaP mediated bio-modification process also opens a wide way for developing various PHA-based targeting systems by presenting different tumor or other tissue-specific targeting peptides.

The fight for invincibility: Environmental stress response mechanisms and Aeromonas hydrophila

Aeromonas hydrophila is a freshwater-dwelling zoonotic bacterium that has economic importance in aquaculture. In the past decade, Aeromonas hydrophila has become increasingly important because of its emergence as a food-borne zoonotic pathogen that is resistant to different treatment regimes. Being an aquatic bacterium, Aeromonas hydrophila is frequently subjected to several stressful environmental conditions, including changes in temperature, acidic pH and starvation that challenge its survival. To cope with these stressful conditions, like every cell, A. hydrophila possesses stress response mechanisms, such as alternative sigma factors, two-component systems, heat shock proteins, cold shock proteins, and acid tolerance response systems that eventually lead the fittest to survive. Moreover, the establishment of genetic variations among the strains related to environmental stress is also of great concern. This review presents the understandings based on inter-strain variations and stress response behavior of A. hydrophila that are important to control the increasing outbreaks of this bacterium in both human populations and aquaculture.

Polyhydroxyalkanoate-associated phasins as phylogenetically heterogeneous, multipurpose proteins

Polyhydroxyalkanoates (PHAs) are natural polyesters of increasing biotechnological importance that are synthesized by many prokaryotic organisms as carbon and energy storage compounds in limiting growth conditions. PHAs accumulate intracellularly in form of inclusion bodies that are covered with a proteinaceous surface layer (granule-associated proteins or GAPs) conforming a network-like surface of structural, metabolic and regulatory polypeptides, and configuring the PHA granules as complex and well-organized subcellular structures that have been designated as 'carbonosomes'. GAPs include several enzymes related to PHA metabolism (synthases, depolymerases and hydroxylases) together with the so-called phasins, an heterogeneous group of small-size proteins that cover most of the PHA granule and that are devoid of catalytic functions but nevertheless play an essential role in granule structure and PHA metabolism. Structurally, phasins are amphiphilic proteins that shield the hydrophobic polymer from the cytoplasm. Here, we summarize the characteristics of the different phasins identified so far from PHA producer organisms and highlight the diverse opportunities that they offer in the Biotechnology field.

Modelling of microbial polyhydroxyalkanoate surface binding protein PhaP for rational mutagenesis

Phasins are unusual amphiphilic proteins that bind to microbial polyhydroxyalkanoate (PHA) granules in nature and show great potential for various applications in biotechnology and medicine. Despite their remarkable diversity, only the crystal structure of PhaP_Ah from Aeromonas hydrophila has been solved to date. Based on the structure of PhaP_Ah, homology models of PhaP_Az from Azotobacter sp. FA‐8 and PhaP_TD from Halomonas bluephagenesis TD were successfully established, allowing rational mutagenesis to be conducted to enhance the stability and surfactant properties of these proteins. PhaP_Az mutants, including PhaP_AzQ38L and PhaP_AzQ78L, as well as PhaP_TD mutants, including PhaP_TDQ38M and PhaP_TDQ72M, showed better emulsification properties and improved thermostability (6‐10°C higher melting temperatures) compared with their wild‐type homologues under the same conditions. Importantly, the established PhaP homology‐modelling approach, based on the high‐resolution structure of PhaP_Ah, can be generalized to facilitate the study of other PhaP members.