Polyketide synthase genes and the natural products potential of Dictyostelium discoideum

The chemical ecology and physiological functions of type I polyketide natural products: the emerging picture

Covering: up to 2024.For many years, the value of complex polyketides lay in their medical properties, including their antibiotic and antifungal activities, with little consideration paid to their native functions. However, more recent evidence gathered from the study of inter-organismal interactions has revealed the influence of these metabolites upon the ecological adaptation and distribution of their hosts, as well as their modes of communication. The increasing number of sequenced genomes and associated transcriptomes has also unveiled the widespread occurrence of the underlying biosynthetic enzymes across all kingdoms of life, and the important contributions they make to physiological events specific to each organism. This review depicts the diversity of roles fulfilled by type I polyketides, particularly in light of studies carried out during the last decade, providing an initial overall picture of their diverse functions.

NOVEL INVENTION OF SPORE INDUCTION IN A SISTER SPECIES TO GROUP 4 DICTYOSTELIA

Background

Dictyostelia are soil amoebas that aggregate to form fruiting bodies with spores and stalk cells in response to starvation. Where known, species across the dictyostelid phylogeny use secreted cAMP, detected by cAMP receptors (cARs) to induce the differentiation of spores and to organize fruiting body construction. However, recent deletion of the single cAR of Polyspondylium violaceum (Pvio) left both its fruiting bodies and spores intact.

Methods

To investigate whether Pvio sporulation can occur in the absence of secreted cAMP and to explore alternative inducers in a bioassay , three prespore genes were identified and gene fusions of their promoters with the LacZ reporter gene were transformed into Pvio cells. After assessing the spatial expression pattern of the genes and the stage at which prespore gene expression initiated, the effect of cAMP and other Dictyostelium discoideum ( Ddis) signal molecules were tested on prespore gene expression in vitro.

Results

Pvio genes g4562 (psp1), g2696 (psp2) and g2380 (psp3) were identified as homologs of Ddis spore coat genes. They were first expressed around 4 h of starvation in aggregation centres and later in the posterior 4/5 th of emerging sorogens and the spore head of early fruiting bodies. Cells from dissociated 4 h aggregates and shaken in suspension for 6 h increased prespore- LacZ reporter activity 4-fold for psp1 and 6-fold for psp2, but this increase was at least 5-fold higher when cells were plated on solid substratum for 6 h to develop normally. cAMP had no effect on prespore gene induction and neither had the Pvio chemoattractant glorin nor the Ddis chemoattractants and differentiation inducers folate, c-di-GMP, DIF-1, GABA, cGMP and 8Br-cAMP.

Conclusions

The Pvio lineage uniquely evolved a novel genetic network for synthesis, detection and processing of the signal that triggers its main survival strategy.

Yellow polyketide pigment suppresses premature hatching in social amoeba

Low-molecular-weight natural products from microbes are indispensable in the development of potent drugs. However, their biological roles within an ecological context often remain elusive. Here, we shed light on natural products from eukaryotic microorganisms that have the ability to transition from single cells to multicellular organisms: the social amoebae. These eukaryotes harbor a large number of polyketide biosynthetic genes in their genomes, yet virtually none of the corresponding products can be isolated or characterized. Using complementary molecular biology approaches, including CRISPR-Cas9, we generated polyketide synthase (pks5) inactivation and overproduction strains of the social amoeba Dictyostelium discoideum. Differential, untargeted metabolomics of wild-type versus mutant fruiting bodies allowed us to pinpoint candidate metabolites derived from the amoebal PKS5. Extrachromosomal expression of the respective gene led to the identification of a yellow polyunsaturated fatty acid. Analysis of the temporospatial production pattern of this compound in conjunction with detailed bioactivity studies revealed the polyketide to be a spore germination suppressor.

Scale-up of an amoeba-based process for the production of the cannabinoid precursor olivetolic acid

Background

The availability of new biological platform organisms to get access to innovative products and processes is fundamental for the progress in biotechnology and bioeconomy. The amoeba Dictyostelium discoideum represents a novel host system that has recently been employed for both the discovery of new natural products and as a cell factory for the production of bioactive compounds such as phytochemicals. However, an essential parameter to evaluate the potential of a new host system is the demonstration of its scalability to allow industrial applicability. Here, we aimed to develop a bioprocess for the production of olivetolic acid, the main precursor of cannabinoids synthesized by a recently engineered D. discoideum strain.

Results

In this study, a sophisticated approach is described to scale-up an amoeba-based polyketide production process in stirred tank bioreactors. Due to the shear sensitivity of the cell wall lacking amoebae, the maximum local energy dissipation rate (ε_max) was selected as a measure for the hydromechanical stress level among different scales. By performing 1.6-L scale batch fermentations with different stress conditions, we determined a maximum tolerable ε_max of 3.9 W/kg for D. discoideum. Further, we used this parameter as scale-up criterion to develop a bioprocess for olivetolic acid production starting from a 7-L stirred tank reactor to the industrially relevant 300-L scale with a product concentration of 4.8 µg/L, a productivity of 0.04 µg/L/h and a yield of 0.56 µg/g glucose.

Conclusion

We developed a robust and reliable scale-up strategy for amoeba-based bioprocesses and evaluated its applicability for the production of the cannabinoid precursor olivetolic acid. By determining the maximum tolerable hydromechanical stress level for D. discoideum, we were able to scale-up the process from shake flasks to the 300-L stirred tank reactor without any yield reduction from cell shearing. Hence, we showed the scalability and biotechnological exploitation of amoeba-based processes that can provide a reasonable alternative to chemical syntheses or extractions of phytochemicals from plant biomass.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01943-w.

The polyketide synthase StlA is involved in inducing aggregation in Polysphondylium violaceum

In the social amoeba Dictyostelium discoideum, the polyketide MPBD (4-methyl-5-pentylbenzene-1,3-diol) regulates the gene expressions of cAMP signaling to make cells aggregation-competent and also induces spore maturation. The polyketide synthase StlA is responsible for MPBD biosynthesis in D. discoideum and appears to be conserved throughout the major groups of the social amoeba (Dictyostelia). In this study, we analyzed the function of StlA in Polysphondylium violaceum by identifying the gene sequence and creating the knockout mutants. We found that Pv-stlA- mutants had defects only in cell aggregation but not in spore maturation, indicating that the function of StlA in inducing spore maturation is species-specific. We also found that MPBD could rescue the aggregation defect in Pv-stlA- mutants whereas the mutants normally exhibited chemotaxis to their chemoattractant, glorin. Our data suggest that StlA is involved in inducing aggregation in P. violaceum by acting on signaling pathways other than chemotaxis in P. violaceum.

The potential of amoeba-based processes for natural product syntheses

The identification of novel platform organisms for the production and discovery of small molecules is of high interest for the pharmaceutical industry. In particular, the structural complexity of most natural products with therapeutic potential restricts an industrial production since chemical syntheses often require complex multistep routes. The amoeba Dictyostelium discoideum can be easily cultivated in bioreactors due to its planktonic growth behavior and contains numerous polyketide and terpene synthase genes with only a few compounds being already elucidated. Hence, the amoeba both bears a wealth of hidden natural products and allows for the development of new bioprocesses for existing pharmaceuticals. In this mini review, we present D. discoideum as a novel platform for the production of complex secondary metabolites and discuss its suitability for industrial processes. We also provide initial insights into future bioprocesses, both involving bacterial coculture setups and for the production of plant-based pharmaceuticals.

Engineering the amoeba Dictyostelium discoideum for biosynthesis of a cannabinoid precursor and other polyketides

Aromatic polyketides are natural polyphenolic compounds with a broad spectrum of pharmacological activities. Production of those metabolites in the model organisms Escherichia coli and Saccharomyces cerevisiae has been limited by the extensive cellular engineering needed for the coordinated biosynthesis of polyketides and their precursors. In contrast, the amoeba Dictyostelium discoideum is a native producer of secondary metabolites and harbors a wide, but largely unexplored, repertoire of genes for the biosynthesis of polyketides and terpenoids. Here we present D. discoideum as an advantageous chassis for the production of aromatic polyketides. By expressing its native and cognate plant polyketide synthase genes in D. discoideum, we demonstrate production of phlorocaprophenone, methyl-olivetol, resveratrol and olivetolic acid (OA), which is the central intermediate in the biosynthesis of cannabinoids. To facilitate OA synthesis, we further engineered an amoeba/plant inter-kingdom hybrid enzyme that produced OA from primary metabolites in two enzymatic steps, providing a shortcut in a synthetic cannabinoid pathway using the D. discoideum host system.

Phospholipids containing ether-bound hydrocarbon-chains are essential for efficient phagocytosis and neutral lipids of the ester-type perturb development in Dictyostelium

Lipids are the building blocks for cellular membranes; they provide signalling molecules for membrane dynamics and serve as energy stores. One path of their synthesis is initiated by glycerol-3-phosphate acyltransferase (GPAT), which in Dictyostelium resides on the endoplasmic reticulum. When an excess of fatty acids is present, it redistributes to storage organelles, the lipid droplets. Mutants, where the GPAT was eliminated by homologous recombination, produce fewer lipid droplets and are almost devoid of triacylglycerols (TAG), rendering them more resistant to cell death and cell loss in the developmental stages preceding fruiting body formation. The enzyme most closely related to GPAT is called FARAT, because it combines a fatty acyl-reductase (FAR) and an acyltransferase (AT) domain in its sequence. The protein is confined to the lumen of the peroxisome, where it transfers a fatty acid to dihydroxyacetone-phosphate initiating the synthesis of ether lipids, later completed at the endoplasmic reticulum. A mutant lacking FARAT produces lipid droplets that are devoid of the storage lipid monoalkyl-diacyl-glycerol (MDG), but the efficiency of spore formation in the developmental cycle is largely unaltered. Instead, these mutants are strongly impaired in phagocytosis of yeast particles, which is attributed to reduced synthesis of membrane phospholipids containing ether-linked chains.

Insight into the Lifestyle of Amoeba Willaertia magna during Bioreactor Growth Using Transcriptomics and Proteomics

Willaertia magna C2c maky is a thermophilic free-living amoeba strain that showed ability to eliminate Legionella pneumophila, a pathogenic bacterium living in the aquatic environment. The amoeba industry has proposed the use of Willaertia magna as a natural biocide to control L. pneumophila proliferation in cooling towers. Here, transcriptomic and proteomic studies were carried out in order to expand knowledge on W. magna produced in a bioreactor. Illumina RNA-seq generated 217 million raw reads. A total of 8790 transcripts were identified, of which 6179 and 5341 were assigned a function through comparisons with National Center of Biotechnology Information (NCBI) reference sequence and the Clusters of Orthologous Groups of proteins (COG) databases, respectively. To corroborate these transcriptomic data, we analyzed the W. magna proteome using LC–MS/MS. A total of 3561 proteins were identified. The results of transcriptome and proteome analyses were highly congruent. Metabolism study showed that W. magna preferentially consumed carbohydrates and fatty acids to grow. Finally, an in-depth analysis has shown that W. magna produces several enzymes that are probably involved in the metabolism of secondary metabolites. Overall, our multi-omic study of W. magna opens the way to a better understanding of the genetics and biology of this amoeba.

Proteomic and Transcriptomic Profiling Identifies Early Developmentally Regulated Proteins in Dictyostelium Discoideum

Cyclic AMP acts as a secondary messenger involving different cellular functions in eukaryotes. Here, proteomic and transcriptomic profiling has been combined to identify novel early developmentally regulated proteins in eukaryote cells. These proteomic and transcriptomic experiments were performed in Dictyostelium discoideum given the unique advantages that this organism offers as a eukaryotic model for cell motility and as a nonmammalian model of human disease. By comparing whole-cell proteome analysis of developed (cAMP-pulsed) wild-type AX2 cells and an independent transcriptomic analysis of developed wild-type AX4 cells, our results show that up to 70% of the identified proteins overlap in the two independent studies. Among them, we have found 26 proteins previously related to cAMP signaling and identified 110 novel proteins involved in calcium signaling, adhesion, actin cytoskeleton, the ubiquitin-proteasome pathway, metabolism, and proteins that previously lacked any annotation. Our study validates previous findings, mostly for the canonical cAMP-pathway, and also generates further insight into the complexity of the transcriptomic changes during early development. This article also compares proteomic data between parental and cells lacking glkA, a GSK-3 kinase implicated in substrate adhesion and chemotaxis in Dictyostelium. This analysis reveals a set of proteins that show differences in expression in the two strains as well as overlapping protein level changes independent of GlkA.

A terpene synthase-cytochrome P450 cluster in Dictyostelium discoideum produces a novel trisnorsesquiterpene

Terpenoids are enormously diverse, but our knowledge of their biosynthesis and functions is limited. Here we report on a terpene synthase (DdTPS8)-cytochrome P450 (CYP521A1) gene cluster that produces a novel C12 trisnorsesquiterpene and affects the development of Dictyostelium discoideum. DdTPS8 catalyzes the formation of a sesquiterpene discoidol, which is undetectable from the volatile bouquet of wild type D. discoideum. Interestingly, a DdTPS8 knockout mutant lacks not only discoidol, but also a putative trisnorsesquiterpene. This compound was hypothesized to be derived from discoidol via cytochrome P450 (CYP)-catalyzed oxidative cleavage. CYP521A1, which is clustered with DdTPS8, was identified as a top candidate. Biochemical assays demonstrated that CYP521A1 catalyzes the conversion of discoidol to a novel trisnorsesquiterpene named discodiene. The DdTPS8 knockout mutant exhibited slow progression in development. This study points to the untapped diversity of natural products made by D. discoideum, which may have diverse roles in its development and chemical ecology.

Diversity and Functional Evolution of Terpene Synthases in Dictyostelid Social Amoebae

Dictyostelids, or social amoebae, have a unique life style in forming multicellular fruiting bodies from unicellular amoeboids upon starvation. Recently, dictyostelids were found to contain terpene synthase (TPS) genes, a gene type of secondary metabolism previously known to occur only in plants, fungi and bacteria. Here we report an evolutionary functional study of dictyostelid TPS genes. The number of TPS genes in six species of dictyostelids examined ranges from 1 to 19; and the model species Dictyostelium purpureum contains 12 genes. Using in vitro enzyme assays, the 12 TPS genes from D. purpureum were shown to encode functional enzymes with distinct product profiles. The expression of the 12 TPS genes in D. purpureum is developmentally regulated. During multicellular development, D. purpureum releases a mixture of volatile terpenes dominated by sesquiterpenes that are the in vitro products of a subset of the 12 TPS genes. The quality and quantity of the terpenes released from D. purpureum, however, bear little resemblance to those of D. discoideum, a closely related dictyostelid. Despite these variations, the conserved clade of dictyostelid TPSs, which have an evolutionary distance of more than 600 million years, has the same biochemical function, catalyzing the formation of a sesquiterpene protoillud-7-ene. Taken together, our results indicate that the dynamic evolution of dictyostelid TPS genes includes both purifying selection of an orthologous group and species-specific expansion with functional divergence. Consequently, the terpenes produced by these TPSs most likely have conserved as well as species-adaptive biological functions as chemical languages in dictyostelids.

Natural Products from Social Amoebae

Natural products are invaluable sources of structural diversity and complexity ideally suited for the development of therapeutic agents. The search for novel bioactive molecules has prompted scientists to explore various ecological niches. Microorganisms have been shown to constitute such an important source. Despite their biosynthetic potential, social amoebae, that is, microorganisms with both a uni- and multicellular lifestyle, are underexplored regarding their secreted secondary metabolome. In this review, we present the structural diversity of amoebal natural products and discuss their biological functions as well as their total syntheses.

Effects of deletion of the receptor CrlA on Dictyostelium aggregation and MPBD-mediated responses are strain dependent and not evident in strain Ax2

The polyketide MPBD (4-methyl-5-pentylbenzene-1, 3-diol) is produced by the polyketide synthase SteelyA (StlA) in Dictyostelium discoideum. MPBD is required for appropriate expression of cAMP signalling genes involved in cell aggregation and additionally induces the spore maturation at the fruiting body stage. The MPBD signalling pathway for regulation of cell aggregation is unknown, but MPBD effects on sporulation were reported to be mediated by the G-protein coupled receptor CrlA in D. discoideum KAx3. In this study, we deleted the crlA gene from the same parental strain (Ax2) that was used to generate the MPBD-less mutant. We found that unlike the MPBD-less mutant, Ax2-derived crlA- mutants exhibited normal cell aggregation, indicating that in Ax2 MPBD effects on early development do not require CrlA. We also found that the Ax2/crlA- mutant formed normal spores in fruiting bodies. When transformed with PkaC, both Ax2 and Ax2/crlA- similarly responded to MPBD in vitro with spore encapsulation. Our data make it doubtful that CrlA acts as the receptor for MPBD signalling during the development of D. discoideum Ax2.

Synthesis and SAR of 4-methyl-5-pentylbenzene-1,3-diol (MPBD), produced by Dictyostelium discoideum

4-Methyl-5-pentylbenzene-1,3-diol (MPBD) is a secondary metabolite of SteelyA polyketide synthase, which controls cell aggregation and spore maturation of Dictyostelium discoideum. In this study, chemical synthesis of MPBD and its derivatives was achieved. Structure-activity relationship (SAR) studies for antimicrobial activities against Escherichia coli and Bacillus subtilis were also conducted.

Comparative genome and transcriptome analyses of the social amoeba Acytostelium subglobosum that accomplishes multicellular development without germ-soma differentiation

BACKGROUND

Social amoebae are lower eukaryotes that inhabit the soil. They are characterized by the construction of a starvation-induced multicellular fruiting body with a spore ball and supportive stalk. In most species, the stalk is filled with motile stalk cells, as represented by the model organism Dictyostelium discoideum, whose developmental mechanisms have been well characterized. However, in the genus Acytostelium, the stalk is acellular and all aggregated cells become spores. Phylogenetic analyses have shown that it is not an ancestral genus but has lost the ability to undergo cell differentiation.

RESULTS

We performed genome and transcriptome analyses of Acytostelium subglobosum and compared our findings to other available dictyostelid genome data. Although A. subglobosum adopts a qualitatively different developmental program from other dictyostelids, its gene repertoire was largely conserved. Yet, families of polyketide synthase and extracellular matrix proteins have not expanded and a serine protease and ABC transporter B family gene, tagA, and a few other developmental genes are missing in the A. subglobosum lineage. Temporal gene expression patterns are astonishingly dissimilar from those of D. discoideum, and only a limited fraction of the ortholog pairs shared the same expression patterns, so that some signaling cascades for development seem to be disabled in A. subglobosum.

CONCLUSIONS

The absence of the ability to undergo cell differentiation in Acytostelium is accompanied by a small change in coding potential and extensive alterations in gene expression patterns.

c-di-GMP induction of Dictyostelium cell death requires the polyketide DIF-1

Two inducers, DIF-1 and c-di-GMP, each separately shown to play a major role in Dictyostelium cell death induction in vitro, in fact cooperate. A similar cooperation with polyketides might occur for c-di-GMP effects in other situations and organisms, in particular in innate immunity and cell death in animal cells.

Cell death in the model organism Dictyostelium, as studied in monolayers in vitro, can be induced by the polyketide DIF-1 or by the cyclical dinucleotide c-di-GMP. c-di-GMP, a universal bacterial second messenger, can trigger innate immunity in bacterially infected animal cells and is involved in developmental cell death in Dictyostelium. We show here that c-di-GMP was not sufficient to induce cell death in Dictyostelium cell monolayers. Unexpectedly, it also required the DIF-1 polyketide. The latter could be exogenous, as revealed by a telling synergy between c-di-GMP and DIF-1. The required DIF-1 polyketide could also be endogenous, as shown by the inability of c-di-GMP to induce cell death in Dictyostelium HMX44A cells and DH1 cells upon pharmacological or genetic inhibition of DIF-1 biosynthesis. In these cases, c-di-GMP–induced cell death was rescued by complementation with exogenous DIF-1. Taken together, these results demonstrated that c-di-GMP could trigger cell death in Dictyostelium only in the presence of the DIF-1 polyketide or its metabolites. This identified another element of control to this cell death and perhaps also to c-di-GMP effects in other situations and organisms.

The hybrid type polyketide synthase SteelyA is required for cAMP signalling in early Dictyostelium development

BACKGROUND

In our previous study we found that the expression of stlA showed peaks both in the early and last stages of development and that a product of SteelyA, 4-methyl-5-pentylbenzene-1,3-diol (MPBD), controlled Dictyostelium spore maturation during the latter. In this study we focused on the role of SteelyA in early stage development.

PRINCIPAL FINDINGS

Our stlA null mutant showed aggregation delay and abnormally small aggregation territories. Chemotaxis analysis revealed defective cAMP chemotaxis in the stlA null mutant. cAMP chemotaxis was restored by MPBD addition during early stage development. Assay for cAMP relay response revealed that the stlA null mutant had lower cAMP accumulation during aggregation, suggesting lower ACA activity than the wild type strain. Exogenous cAMP pulses rescued the aggregation defect of the stlA null strain in the absence of MPBD. Expression analysis of cAMP signalling genes revealed lower expression levels in the stlA null mutant during aggregation.

CONCLUSION

Our data indicate a regulatory function by SteelyA on cAMP signalling during aggregation and show that SteelyA is indispensable for full activation of ACA.

Quantification of polyketide synthase genes in tropical urban soils using real-time PCR

Polyketide synthases (PKSs) catalyze the biosynthesis of polyketides and may contribute to the natural production of antibiotics and pose selective pressure for the development of antibiotic resistant bacteria in the environment. Although conventional PCR have been developed to detect the presence of PKS genes, no previous studies have been done to quantify the abundance of PKS genes in environmental samples. In this study, two sets of degenerate real-time PCR (qPCR) primers (PKS1-F/PKS1-R, PKS2-F/PKS2-R) with high specificity and sensitivity were developed to quantify PKS type I and type II genes. These primers were subsequently used to quantify PKS genes in tropical urban soils, and both PKS genes were widely detected in all soil samples. The absolute abundance of PKS type I ranged from 1.7×10(6) to 4.7×10(6) copies per gram of soil and the absolute abundance of PKS type II genes ranged from 2.4×10(5) to 1.5×10(6) per gram of soil, and the abundance of PKS type I gene was consistently higher than that of PKS type II gene. The relative abundance of PKS type I gene was positively correlated with that of PKS type II gene (p<0.01). Regression analyses indicate that PKS gene abundance was negatively correlated with environmental factors, such as selected antibiotics, sulfate, and metals (p<0.05), but was not correlated with land use type. The studies on the correlation between environmental factors and PKS genes could provide useful information to understand natural production of antibiotics and its associated environmental risks.

Alkylbenzoquinone involved in development of cellular slime molds

The structure of the prespore-cell-promoting factor from Dictyostelium discoideum was determined to be 2-hydroxy-5-methyl-6-pentylbenzoquinone. The synthetic compound has prespore-cell-promoting activity similar to the natural one, with half-maximal induction at a concentration as low as 40 pM. It was also found that the factor induces aggregation in an aggregation-deficient mutant of a related species, Polysphodilium violaceum. Both these activities are sensitive to positional isomerism with the 6-methyl-5-pentyl isomer showing no detectable activity.

Role of fatty acid synthase in the development of Dictyostelium discoideum

Fatty acids are fundamental cellular components, and provide essential building blocks for membrane biosynthesis. Although the use of gene knockout mutants is a robust method for examining the function of specific cellular metabolic networks, fatty acid synthase knockout mutants are extremely difficult to isolate. In the Dictyostelium discoideum genome, we found two putative fatty acid synthase genes, and we created a knockout mutant for one of them to examine the physiological consequences. In this study, we found that a continuous fatty acid supply was necessary for normal development, and the fatty acid synthase knockout mutant showed severe developmental delay. This developmental defect was corrected in chimeras composed of wild type cells and knockout mutant cells (3:7, respectively). The knockout mutant also showed aberrant expression of fatty acid biosynthesis genes. These results showed that D. discoideum needs correct fatty acid synthesis for normal development.

Phylogeny-wide analysis of social amoeba genomes highlights ancient origins for complex intercellular communication

Dictyostelium discoideum (DD), an extensively studied model organism for cell and developmental biology, belongs to the most derived group 4 of social amoebas, a clade of altruistic multicellular organisms. To understand genome evolution over long time periods and the genetic basis of social evolution, we sequenced the genomes of Dictyostelium fasciculatum (DF) and Polysphondylium pallidum (PP), which represent the early diverging groups 1 and 2, respectively. In contrast to DD, PP and DF have conventional telomere organization and strongly reduced numbers of transposable elements. The number of protein-coding genes is similar between species, but only half of them comprise an identifiable set of orthologous genes. In general, genes involved in primary metabolism, cytoskeletal functions and signal transduction are conserved, while genes involved in secondary metabolism, export, and signal perception underwent large differential gene family expansions. This most likely signifies involvement of the conserved set in core cell and developmental mechanisms, and of the diverged set in niche- and species-specific adaptations for defense and food, mate, and kin selection. Phylogenetic dating using a concatenated data set and extensive loss of synteny indicate that DF, PP, and DD split from their last common ancestor at least 0.6 billion years ago.

Two functionally distinctive phosphopantetheinyl transferases from amoeba Dictyostelium discoideum

The life cycle of Dictyostelium discoideum is proposed to be regulated by expression of small metabolites. Genome sequencing studies have revealed a remarkable array of genes homologous to polyketide synthases (PKSs) that are known to synthesize secondary metabolites in bacteria and fungi. A crucial step in functional activation of PKSs involves their post-translational modification catalyzed by phosphopantetheinyl transferases (PPTases). PPTases have been recently characterized from several bacteria; however, their relevance in complex life cycle of protozoa remains largely unexplored. Here we have identified and characterized two phosphopantetheinyl transferases from D. discoideum that exhibit distinct functional specificity. DiAcpS specifically modifies a stand-alone acyl carrier protein (ACP) that possesses a mitochondrial import signal. DiSfp in contrast is specific to Type I multifunctional PKS/fatty acid synthase proteins and cannot modify the stand-alone ACP. The mRNA of two PPTases can be detected during the vegetative as well as starvation-induced developmental pathway and the disruption of either of these genes results in non-viable amoebae. Our studies show that both PPTases play an important role in Dictyostelium biology and provide insight into the importance of PPTases in lower eukaryotes.

Dictyostelium hybrid polyketide synthase, SteelyA, produces 4-methyl-5-pentylbenzene-1,3-diol and induces spore maturation

The genome of Dictyostelium contains two novel hybrid-type polyketide synthases (PKSs) known as 'Steely'; the Steely enzyme is formed by the fusion of type I and type III PKSs. One of these enzymes, SteelyB, is known to be responsible for the production of the stalk cell-inducing factor DIF-1 in vivo. On the other hand, the product(s) and expression pattern of SteelyA are not clearly understood, because there are two different reports associated with the in vitro products of SteelyA and its expression pattern. To solve this problem, we first examined the expression pattern using two different primer sets and found that it was quite similar to that shown in the dictyExpress database. stlA expression peaked at approximately 3 h and declined, but showed a small peak around the end of development. Next, we examined the in vivo product of SteelyA using a stlA null mutant and found that the mutant lacked 4-methyl-5-pentylbenzene-1,3-diol (MPBD). This null mutant showed aberrant, glassy sori, and most of the cells in the sori remained amoeba-like without a cell wall. This defect was restored by adding 200 nM of MPBD to the agar. These results indicate that SteelyA produces MPBD in vivo and induces spore maturation.

Comparative genomics of the social amoebae Dictyostelium discoideum and Dictyostelium purpureum

BACKGROUND

The social amoebae (Dictyostelia) are a diverse group of Amoebozoa that achieve multicellularity by aggregation and undergo morphogenesis into fruiting bodies with terminally differentiated spores and stalk cells. There are four groups of dictyostelids, with the most derived being a group that contains the model species Dictyostelium discoideum.

RESULTS

We have produced a draft genome sequence of another group dictyostelid, Dictyostelium purpureum, and compare it to the D. discoideum genome. The assembly (8.41 × coverage) comprises 799 scaffolds totaling 33.0 Mb, comparable to the D. discoideum genome size. Sequence comparisons suggest that these two dictyostelids shared a common ancestor approximately 400 million years ago. In spite of this divergence, most orthologs reside in small clusters of conserved synteny. Comparative analyses revealed a core set of orthologous genes that illuminate dictyostelid physiology, as well as differences in gene family content. Interesting patterns of gene conservation and divergence are also evident, suggesting function differences; some protein families, such as the histidine kinases, have undergone little functional change, whereas others, such as the polyketide synthases, have undergone extensive diversification. The abundant amino acid homopolymers encoded in both genomes are generally not found in homologous positions within proteins, so they are unlikely to derive from ancestral DNA triplet repeats. Genes involved in the social stage evolved more rapidly than others, consistent with either relaxed selection or accelerated evolution due to social conflict.

CONCLUSIONS

The findings from this new genome sequence and comparative analysis shed light on the biology and evolution of the Dictyostelia.

Phylogenetic methods in natural product research

Natural products researchers are increasingly employing evolutionary analyses of genes and gene products that rely on phylogenetic trees. The field of phylogenetic inference and of evolutionary analyses based on phylogenies is growing at an amazing rate, making it difficult to keep up with the latest methodologies. Here, we summarize phylogenetic applications in natural products research, and review methods and software useful for carrying out analyses inferring or using phylogenetic trees. We include an updated overview of available alignment methods and programs, as well as a selection of some useful phylogenetic analysis tools. This review covers primarily the period 2000-2009 for applications of phylogenetic methods in natural product research, and 1990-2009 for phylogenetic methods, with some references going back to the 1960s.

ClustScan: an integrated program package for the semi-automatic annotation of modular biosynthetic gene clusters and in silico prediction of novel chemical structures

The program package ‘ClustScan’ (Cluster Scanner) is designed for rapid, semi-automatic, annotation of DNA sequences encoding modular biosynthetic enzymes including polyketide synthases (PKS), non-ribosomal peptide synthetases (NRPS) and hybrid (PKS/NRPS) enzymes. The program displays the predicted chemical structures of products as well as allowing export of the structures in a standard format for analyses with other programs. Recent advances in understanding of enzyme function are incorporated to make knowledge-based predictions about the stereochemistry of products. The program structure allows easy incorporation of additional knowledge about domain specificities and function. The results of analyses are presented to the user in a graphical interface, which also allows easy editing of the predictions to incorporate user experience. The versatility of this program package has been demonstrated by annotating biochemical pathways in microbial, invertebrate animal and metagenomic datasets. The speed and convenience of the package allows the annotation of all PKS and NRPS clusters in a complete Actinobacteria genome in 2–3 man hours. The open architecture of ClustScan allows easy integration with other programs, facilitating further analyses of results, which is useful for a broad range of researchers in the chemical and biological sciences.

A computational screen for type I polyketide synthases in metagenomics shotgun data

BACKGROUND

Polyketides are a diverse group of biotechnologically important secondary metabolites that are produced by multi domain enzymes called polyketide synthases (PKS).

METHODOLOGY/PRINCIPAL FINDINGS

We have estimated frequencies of type I PKS (PKS I) - a PKS subgroup - in natural environments by using Hidden-Markov-Models of eight domains to screen predicted proteins from six metagenomic shotgun data sets. As the complex PKS I have similarities to other multi-domain enzymes (like those for the fatty acid biosynthesis) we increased the reliability and resolution of the dataset by maximum-likelihood trees. The combined information of these trees was then used to discriminate true PKS I domains from evolutionary related but functionally different ones. We were able to identify numerous novel PKS I proteins, the highest density of which was found in Minnesota farm soil with 136 proteins out of 183,536 predicted genes. We also applied the protocol to UniRef database to improve the annotation of proteins with so far unknown function and identified some new instances of horizontal gene transfer.

CONCLUSIONS/SIGNIFICANCE

The screening approach proved powerful in identifying PKS I sequences in large sequence data sets and is applicable to many other protein families.

DIF-1 induces the basal disc of the Dictyostelium fruiting body

The polyketide DIF-1 induces Dictyostelium amoebae to form stalk cells in culture. To better define its role in normal development, we examined the phenotype of a mutant blocking the first step of DIF-1 synthesis, which lacks both DIF-1 and its biosynthetic intermediate, dM-DIF-1 (des-methyl-DIF-1). Slugs of this polyketide synthase mutant (stlB(-)) are long and thin and rapidly break up, leaving an immotile prespore mass. They have approximately 30% fewer prestalk cells than their wild-type parent and lack a subset of anterior-like cells, which later form the outer basal disc. This structure is missing from the fruiting body, which perhaps in consequence initiates culmination along the substratum. The lower cup is rudimentary at best and the spore mass, lacking support, slips down the stalk. The dmtA(-) methyltransferase mutant, blocked in the last step of DIF-1 synthesis, resembles the stlB(-) mutant but has delayed tip formation and fewer prestalk-O cells. This difference may be due to accumulation of dM-DIF-1 in the dmtA(-) mutant, since dM-DIF-1 inhibits prestalk-O differentiation. Thus, DIF-1 is required for slug migration and specifies the anterior-like cells forming the basal disc and much of the lower cup; significantly the DIF-1 biosynthetic pathway may supply a second signal - dM-DIF-1.