Improving Microbial Cell Factory Performance by Engineering SAM Availability

Methylated natural products are widely spread in nature. S-Adenosyl-l-methionine (SAM) is the secondary abundant cofactor and the primary methyl donor, which confer natural products with structural and functional diversification. The increasing demand for SAM-dependent natural products (SdNPs) has motivated the development of microbial cell factories (MCFs) for sustainable and efficient SdNP production. Insufficient and unsustainable SAM availability hinders the improvement of SdNP MCF performance. From the perspective of developing MCF, this review summarized recent understanding of de novo SAM biosynthesis and its regulatory mechanism. SAM is just the methyl mediator but not the original methyl source. Effective and sustainable methyl source supply is critical for efficient SdNP production. We compared and discussed the innate and relatively less explored alternative methyl sources and identified the one involving cheap one-carbon compound as more promising. The SAM biosynthesis is synergistically regulated on multilevels and is tightly connected with ATP and NAD(P)H pools. We also covered the recent advancement of metabolic engineering in improving intracellular SAM availability and SdNP production. Dynamic regulation is a promising strategy to achieve accurate and dynamic fine-tuning of intracellular SAM pool size. Finally, we discussed the design and engineering constraints underlying construction of SAM-responsive genetic circuits and envisioned their future applications in developing SdNP MCFs.

Exploring Mycosporine-like Amino Acid UV-Absorbing Natural Products for a New Generation of Environmentally Friendly Sunscreens

Human skin needs additional protection from damaging ultraviolet radiation (UVR: 280-400 nm). Harmful UVR exposure leads to DNA damage and the development of skin cancer. Available sunscreens offer chemical protection from detrimental sun radiation to a certain extent. However, many synthetic sunscreens do not provide sufficient UVR protection due to the lack of photostability of their UV-absorbing active ingredients and/or the lack of ability to prevent the formation of free radicals, inevitably leading to skin damage. In addition, synthetic sunscreens may negatively affect human skin, causing irritation, accelerating skin aging and even resulting in allergic reactions. Beyond the potential negative effect on human health, some synthetic sunscreens have been shown to have a harmful impact on the environment. Consequently, identifying photostable, biodegradable, non-toxic, and renewable natural UV filters is imperative to address human health needs and provide a sustainable environmental solution. In nature, marine, freshwater, and terrestrial organisms are protected from harmful UVR through several important photoprotective mechanisms, including the synthesis of UV-absorbing compounds such as mycosporine-like amino acids (MAAs). Beyond MAAs, several other promising, natural UV-absorbing products could be considered for the future development of natural sunscreens. This review investigates the damaging impact of UVR on human health and the necessity of using sunscreens for UV protection, specifically UV-absorbing natural products that are more environmentally friendly than synthetic UV filters. Critical challenges and limitations related to using MAAs in sunscreen formulations are also evaluated. Furthermore, we explain how the genetic diversity of MAA biosynthetic pathways may be linked to their bioactivities and assess MAAs' potential for applications in human health.

Halotolerance, stress mechanisms, and circadian clock of salt-tolerant cyanobacteria

Cyanobacteria harbor a high level of physiological flexibility, which enables them to reside in virtually all available environmental niches, including extreme environments. In this review, we summarize the recent advancements in stress mechanisms of salt-tolerant (a.k.a. halotolerant) cyanobacteria. Omics approaches have been extensively employed in recent years to decipher mechanisms of halotolerance and to understand the relevance of halotolerance-associated gene regulatory networks. The vast knowledge from genome mining disclosed that halotolerant cyanobacteria possess extended gene families and/or clusters, encoding enzymes that synthesize unique osmoprotectants, including glycine betaine (GB), betaine derivatives, and mycosporine-like amino acids (MAAs). Comprehensive transcriptomic analyses were conducted using Halothece sp. PCC7418 (hereafter referred to as Halothece), a cyanobacterium that exhibits remarkable halotolerance. These studies revealed a specific transcriptional response when Halothece was subjected to salt stress, whereas salt and osmotic stresses were found to share a common transcriptomic response. Transcriptome and metabolite analyses of Halothece illustrated a complex dynamic relationship between the biosyntheses of osmoprotectants, as well as corresponding and ancillary pathways. Lastly, novel insights highlight the relationship between the molecular regulation of the circadian rhythm and salt stress tolerance. Since the circadian rhythm of gene expression was distorted under salt stress, halotolerant cyanobacteria may prioritize the adaptation to salt stress by attenuation of circadian rhythmicity. KEY POINTS: • Recent advancements in the understanding of stress mechanisms in halotolerant cyanobacteria are described based on omics analyses. • Transcriptome and metabolite analyses of Halothece illustrated a complex dynamic relationship between the biosyntheses of osmoprotectants, as well as corresponding and ancillary pathways. • Since salt stress affects the molecular regulation among clock-related proteins, salt stress may attenuate circadian rhythmicity.

Heterologous Production of Glycine Betaine Using Synechocystis sp. PCC 6803-Based Chassis Lacking Native Compatible Solutes

Among compatible solutes, glycine betaine has various applications in the fields of nutrition, pharmaceuticals, and cosmetics. Currently, this compound can be extracted from sugar beet plants or obtained by chemical synthesis, resulting in low yields or high carbon footprint, respectively. Hence, in this work we aimed at exploring the production of glycine betaine using the unicellular cyanobacterium Synechocystis sp. PCC 6803 as a photoautotrophic chassis. Synechocystis mutants lacking the native compatible solutes sucrose or/and glucosylglycerol-∆sps, ∆ggpS, and ∆sps∆ggpS-were generated and characterized. Under salt stress conditions, the growth was impaired and accumulation of glycogen decreased by ∼50% whereas the production of compatible solutes and extracellular polymeric substances (capsular and released ones) increased with salinity. These mutants were used as chassis for the implementation of a synthetic device based on the metabolic pathway described for the halophilic cyanobacterium Aphanothece halophytica for the production of the compatible solute glycine betaine. Transcription of ORFs comprising the device was shown to be stable and insulated from Synechocystis' native regulatory network. Production of glycine betaine was achieved in all chassis tested, and was shown to increase with salinity. The introduction of the glycine betaine synthetic device into the ∆ggpS background improved its growth and enabled survival under 5% NaCl, which was not observed in the absence of the device. The maximum glycine betaine production [64.29 µmol/gDW (1.89 µmol/mg protein)] was reached in the ∆ggpS chassis grown under 3% NaCl. Taking into consideration this production under seawater-like salinity, and the identification of main key players involved in the carbon fluxes, this work paves the way for a feasible production of this, or other compatible solutes, using optimized Synechocystis chassis in a pilot-scale.

Recent advances in the discovery of novel marine natural products and mycosporine-like amino acid UV-absorbing compounds

Abstract

Bioactive compounds from marine environments represent a rich source of bioproducts for potential use in medicine and biotechnology. To discover and identify novel marine natural products (MNPs), evaluating diverse biological activities is critical. Increased sensitivity and specificity of omics technologies, especially next-generation high-throughput sequencing combined with liquid chromatography-mass spectrometry and nuclear magnetic resonance, are speeding up the discovery of novel bioactive compounds. Mycosporine-like amino acids (MAAs) isolated from many marine microorganisms are among highly promising MNPs characterized by ultraviolet radiation (UV) absorbing capacities and are recognized as a potential source of ecologically friendly sunscreens. MAAs absorb damaging UV radiation with maximum absorption in the range of 310–360 nm, including both UVA and UVB ranges. MAAs are also characterized by other biological activities such as anti-oxidant, anti-cancer, and anti-inflammatory activities. The application of modern omics approaches promoted some recent developments in our understanding of MAAs’ functional significance and diversity. This review will summarize the various modern tools that could be applied during the identification and characterization of MNPs, including MAAs, to further their innovative applications.

Key points

• New omics technologies are speeding up the discovery of novel bio-products

• The vast diversity of bioactive capacities of marine natural products described

• Marine microorganisms as a source of environmentally friendly sunscreens

Insights into the phylogeny and transcriptional response of serine proteases in a halotolerant cyanobacterium Halothece sp. PCC7418

Serine proteases are a class of versatile proteolytic enzymes. They are necessary for protein catabolism, intracellular amino acid turnover, and regulation of proteins involved in diverse molecular and cellular processes across taxa. In this study, bioinformatic analyses revealed a significantly large number of serine proteases in the halotolerant cyanobacterium Halothece sp. PCC7418 (hereafter referred to as Halothece 7418) compared to the model freshwater cyanobacterium Synechococcus elongatus PCC7942 (hereafter referred to as S. elongatus 7942). The cyanobacterial serine proteases are likely derived from different linages since no conserved motifs were detected. The presence of highly diverse serine proteases in Halothece 7418 implicated an evolutionary-mediated modification of several proteases, which may play numerous physiological roles. We also examined the gene expression patterns of 34 serine protease encoding genes in Halothece 7418 exposed to salt stress. Our results revealed that several serine protease genes were drastically up-regulated under salt with high concentration but remained unchanged under salt with low concentration. All four clp genes (H1996, H1997, H0950, and H3375) and H3553 gene (which encodes a putative HtrA protease) were significantly induced upon salt stress. These responses support the roles of the housekeeping pathways in both the degradation of damaged proteins induced by salt stress and regulation of proteins involved in the molecular recovery from salt stress. Since serine proteases share several biochemical features and physiological functions, the results from this study provide an insight into diversification of serine proteases in cyanobacteria. Further, these results will increase our understanding of several mechanisms at the subcellular level.

Mycosporine-like amino acid and aromatic amino acid transcriptome response to UV and far-red light in the cyanobacterium Chlorogloeopsis fritschii PCC 6912

The "UV sunscreen" compounds, the mycosporine-like amino acids (MAAs) are widely reported in cyanobacteria and are known to be induced under ultra-violet (UV) light. However, the impact of far red (FR) light on MAA biosynthesis has not been studied. We report results from two experiments measuring transcriptional regulation of MAA and aromatic amino acid pathways in the filamentous cyanobacterium Chlorogloeopsis fritschii PCC 6912. The first experiment, comparing UV with white light, shows the expected upregulation of the characteristic MAA mys gene cluster. The second experiment, comparing FR with white light, shows that three genes of the four mys gene cluster encoding up to mycosporine-glycine are also upregulated under FR light. This is a new discovery. We observed corresponding increases in MAAs under FR light using HPLC analysis. The tryptophan pathway was upregulated under UV, with no change under FR. The tyrosine and phenylalanine pathways were unaltered under both conditions. However, nitrate ABC transporter genes were upregulated under UV and FR light indicating increased nitrogen requirement under both light conditions. The discovery that MAAs are upregulated under FR light supports MAAs playing a role in photon dissipation and thermoregulation with a possible role in contributing to Earth surface temperature regulation.

Expression of a stress-responsive gene cluster for mycosporine-2-glycine confers oxidative stress tolerance in Synechococcus elongatus PCC7942

Mycosporine-like amino acids (MAAs) are a class of well-documented UV-screening compounds produced by taxonomically diverse organisms. Extensive studies revealed that a rare MAA, mycosporine-2-glycine (M2G), possesses unique biological activities and functions. M2G is not only a potent antioxidant, but also suppresses protein glycation in vitro, and production of inflammatory mediators in RAW 264.7 macrophages. The present study evaluates vital functions of M2G in a heterologous expression system. The stress-sensitive fresh water cyanobacterium Synechococcus elongatus PCC7942, carrying a M2G biosynthetic gene cluster, was generated. The M2G-expressing cells were more tolerant to H2O2-induced oxidative stress than the wild type, with a half-maximal inhibitory concentration (IC50) value of 2.3 ± 0.06 mM. Transcriptional analysis revealed that all M2G biosynthetic genes were highly up-regulated under oxidative stress. Further, expression of vital genes in the cellular antioxidant defense system, including sodB, cat and tpxA were modulated and up-regulated. Elevated M2G was detected under oxidative stress as well as salt stress treatments. This study provides insight into the molecular and cellular effects of the M2G biosynthetic gene cluster, contributing to understanding of the mechanism behind physiological plasticity under this heterologous expression system.

The Effects of Salts and Osmoprotectants on Enzyme Activities of Fructose-1,6-biphosphate Aldolases in a Halotolerant Cyanobacterium, Halothece sp. PCC 7418

The halotolerant cyanobacterium, Halothece sp. PCC 7418, possesses two classes of fructose-1,6-bisphosphate aldolase (FBA): H2846 and H2847. Though class I (CI)-FBA H2846 is thought to be associated with salt tolerance, the regulatory mechanisms, molecular characteristics, and expression profiles between H2846 and class II (CII)-FBA H2847 have scarcely been investigated. Here, we show that the accumulation of the H2846 protein is highly responsive to both up- and down-shock with NaCl, whereas H2847 is constitutively expressed. The activity of CI- and CII-FBA in cyanobacterial extracts is correlated with the accumulation patterns of H2846 and H2847, respectively. In addition, it was found that these activities were inhibited by NaCl and KCl, with CII-FBA activity strikingly inhibited. It was also found that the CI-FBA activity of recombinant H2846 was hindered by salts and that this hindrance could be moderated by the addition of glycine betaine (GB), whereas no moderation occurred with other potential osmoprotectant molecules (proline, sucrose, and glycerol). In addition, a phylogenetic analysis showed that CI-FBAs with higher similarities to H2846 tended to be distributed among potential GB-synthesizing cyanobacteria. Taken together, our results provide insights into the independent evolution of the CI- and CII-FBA gene families, which show distinct expression profiles and functions following salt stress.

Halophile, an essential platform for bioproduction

Industrial biotechnology aims to compete as a stronger alternative ensuring environmental friendly microbial-based production that seeks to curb the predicament of pollution. However, the high cost of bioprocessing is a severe drawback, and therefore, new approaches must be developed to overcome this challenge. Halophiles have shown potentials of overcoming this challenge and are of much preference for unsterile and continuous contamination-free bioprocess due to their unique ability to grow under harsh environmental conditions. Recent advances in genetic manipulations have been established to better the performance of halophiles for industrial applications. Many researchers produced various products such as polyhydroxyalkanoates (PHA), ectoines, biosurfactants, and antioxidants using halophiles, and further efforts have been established to develop halophiles as the foundation for low-cost bioprocess. This paper provides a useful reference for researchers on the merits, drawbacks, achievements, and application of halophiles for bioproduction.

A class I fructose-1,6-bisphosphate aldolase is associated with salt stress tolerance in a halotolerant cyanobacterium Halothece sp. PCC 7418

Fructose-1,6-bisphosphate aldolase (FBA) is a key metabolic enzyme, which is involved in glycolysis, gluconeogenesis and the Calvin cycle. The distinct physiological roles of FBAs in various organisms have been reported; however, in cyanobacteria, the functional characterization of FBAs and investigation of the intracellular dynamics of FBAs largely remains unknown. Here, we utilized a two-step chromatographic technique to identify a class I FBA (CI-FBA), which we named H2846. H2846 was induced by salt stress in the halotolerant cyanobacterium Halothece sp. PCC 7418 (hereafter referred to as Halothece 7418). Phylogenetic analysis showed that H2846-like CI-FBAs existed mainly in cyanobacterial species that inhabit hypersaline environments. Subcellular fractionation revealed that H2846 localized in the cytosolic and periplasmic spaces and size-exclusion chromatography suggested that H2846 formed a homohexamer. The CI-FBA activity of recombinant H2846-mediated cleavage of fructose bisphosphate (FBP) was characterized using a coupled enzymatic assay. This analysis allowed us to determine the K_m and V_max values of recombinant H2846, which were then compared to previously reported K_m and V_max values of several FBAs. Our data suggested that H2846 was likely responsible for the salt stress-induced CI-FBA activity from the total soluble protein extracts derived from Halothece 7418 cells. Moreover, heterologous expression of H2846 but not H2847, a class II FBA (CII-FBA), conferred salt stress tolerance to the salt-sensitive freshwater cyanobacterium, Synechococcus elongatus PCC 7942, which only contains the CII-FBA, S1443. S. elongatus PCC 7942 with a S1443 gene deletion was complemented by H2847 expression, but was not complemented by expression of H2846. Taken together, these results indicate the functional differences between two distinct sets of FBAs in cyanobacteria. H2846 is an active CI-FBA that contributes to the mechanism of salt stress tolerance in Halothece 7418.

Induction of Antioxidative Activity and Antioxidant Molecules in the Halotolerant CyanobacteriumHalothecesp. PCC7418 by Temperature Shift

Extremophiles are a rich source of novel secondary metabolites and valuable antioxidative compounds. Here, we examined the antioxidative capacities of aqueous extracts derived from the halotolerant cyanobacterium Halothece sp. PCC7418. The antioxidative activities of extracts derived from Halothece sp. PCC7418 cells exposed to temperature shock were significantly enhanced. Quantification of antioxidant molecules (phenolic compounds and phycobiliproteins) in the aqueous extracts revealed that the amounts of these molecules were modulated by temperature shock. In addition, the intracellular accumulation of mycosporine-2-glycine, a potential antioxidative molecule, was strongly enhanced by cold shock. Our results show that the treatment of Halothece sp. PCC7418 cells with temperature shock may allow for the robust production of antioxidants.

Antioxidative, Anti-Inflammatory, and Anti-Aging Properties of Mycosporine-Like Amino Acids: Molecular and Cellular Mechanisms in the Protection of Skin-Aging

Prolonged exposure to ultraviolet (UV) radiation causes photoaging of the skin and induces a number of disorders, including sunburn, fine and coarse wrinkles, and skin cancer risk. Therefore, the application of sunscreen has gained much attention to reduce the harmful effects of UV irradiation on our skin. Recently, there has been a growing demand for the replacement of chemical sunscreens with natural UV-absorbing compounds. Mycosporine-like amino acids (MAAs), promising alternative natural UV-absorbing compounds, are a group of widely distributed, low molecular-weight, water-soluble molecules that can absorb UV radiation and disperse the absorbed energy as heat, without generating reactive oxygen species (ROS). More than 30 MAAs have been characterized, from a variety of organisms. In addition to their UV-absorbing properties, there is substantial evidence that MAAs have the potential to protect against skin aging, including antioxidative activity, anti-inflammatory activity, inhibition of protein-glycation, and inhibition of collagenase activity. This review will provide an overview of MAAs, as potential anti-aging ingredients, beginning with their structure, before moving on to discuss the most recent experimental observations, including the molecular and cellular mechanisms through which MAAs might protect the skin. In particular, we focus on the potential anti-aging activity of mycosporine-2-glycine (M2G).

The sedoheptulose 7-phosphate cyclases and their emerging roles in biology and ecology

Covering up to: 1999-2016This highlight covers a family of enzymes of growing importance, the sedoheptulose 7-phosphate cyclases, initially of interest due to their involvement in the biosynthesis of pharmaceutically relevant secondary metabolites. More recently, these enzymes have been found throughout Prokarya and Eukarya, suggesting their broad potential biological roles in nature.

Effect of UV-B Radiation and Desiccation Stress on Photoprotective Compounds Accumulation in Marine Leptolyngbya sp

Increased awareness regarding the harmful effects of ultraviolet (UV)-B radiation has led to the search for new sources of natural UV-B protecting compounds. Mycosporine-like amino acids are one of such promising compounds found in several organisms. Cyanobacteria are ideal organisms for isolation of these compounds due to their compatibility and adaptability to thrive under harsh environmental conditions. In the following investigation, we report the production of shinorine in Leptolyngbya sp. isolated from the intertidal region. Based on the spectral characteristics and liquid chromatography-mass spectrometry analysis, the UV-absorbing compound was identified as shinorine. To the best of our knowledge, this is the first report on the occurrence of shinorine in Leptolyngbya sp. We also investigated the effect of artificial UV-B radiation and periodic desiccation on chlorophyll-a, total carotenoids, and mycosporine-like amino acids (MAAs) production. The UV-B radiation had a negative effect on growth and chlorophyll concentration, whereas it showed an inductive effect on the production of total carotenoids and MAAs. Desiccation along with UV-B radiation led to an increase in the concentration of photoprotective compounds. These results indicate that carotenoids and MAAs thus facilitate cyanobacteria to avoid and protect themselves from the deleterious effects of UV-B and desiccation.

DNA damage protecting and free radical scavenging properties of mycosporine-2-glycine from the Dead Sea cyanobacterium in A375 human melanoma cell lines

Mycosporine-like amino acids (MAAs) are a group of natural sunscreen compounds that possess highly photoprotective properties. The most commonly found MAAs in marine organisms is shinorine, porphyra-334, and mycosporine-glycine. However, the halophilic species accumulate mycosporine-2-glycine (M2G) as the major MAA. In this study, we have investigated the protective effect of M2G against oxidative stress. In vitro radical scavenging activity revealed that M2G exhibited a significant inhibition with scavenging concentration (SC) 50 value of 22±1.4μM. In vivo analysis using the human melanoma A375 and a control cell line (NHSF) showed that M2G at low concentration (i.e. micromolar range) protected the cells against the oxidative stress (H₂O₂)-induced cell death. Comet assay to assess total DNA strand breaks demonstrated that M2G was not genotoxic and protected against the DNA damage by H₂O₂ treatment at the same level as ascorbic acid. To our knowledge, this is the first evidence demonstrating potential protective role of the natural sunscreen compound M2G against oxidative stress-induced DNA damage in human cell lines. The potent antioxidant activity combined with DNA protection ability of M2G may support its endorsement as a potential natural sunscreen with antioxidant property. These findings provide important clues for possible use of M2G in pharmaceutical and biotechnological applications.