Crystal Structure and Function of PqqF Protein in the Pyrroloquinoline Quinone Biosynthetic Pathway

Detoxifying bacterial genes for deoxynivalenol epimerization confer durable resistance to Fusarium head blight in wheat

Fusarium head blight (FHB) and the presence of mycotoxin deoxynivalenol (DON) pose serious threats to wheat production and food safety worldwide. DON, as a virulence factor, is crucial for the spread of FHB pathogens on plants. However, germplasm resources that are naturally resistant to DON and DON-producing FHB pathogens are inadequate in plants. Here, detoxifying bacteria genes responsible for DON epimerization were used to enhance the resistance of wheat to mycotoxin DON and FHB pathogens. We characterized the complete pathway and molecular basis leading to the thorough detoxification of DON via epimerization through two sequential reactions in the detoxifying bacterium Devosia sp. D6-9. Epimerization efficiently eliminates the phytotoxicity of DON and neutralizes the effects of DON as a virulence factor. Notably, co-expressing of the genes encoding quinoprotein dehydrogenase (QDDH) for DON oxidation in the first reaction step, and aldo-keto reductase AKR13B2 for 3-keto-DON reduction in the second reaction step significantly reduced the accumulation of DON as virulence factor in wheat after the infection of pathogenic Fusarium, and accordingly conferred increased disease resistance to FHB by restricting the spread of pathogenic Fusarium in the transgenic plants. Stable and improved resistance was observed in greenhouse and field conditions over multiple generations. This successful approach presents a promising avenue for enhancing FHB resistance in crops and reducing mycotoxin contents in grains through detoxification of the virulence factor DON by exogenous resistance genes from microbes.

New Insights into Pseudomonas spp.-Produced Antibiotics: Genetic Regulation of Biosynthesis and Implementation in Biotechnology

Pseudomonas bacteria are renowned for their remarkable capacity to synthesize antibiotics, namely mupirocin, gluconic acid, pyrrolnitrin, and 2,4-diacetylphloroglucinol (DAPG). While these substances are extensively employed in agricultural biotechnology to safeguard plants against harmful bacteria and fungi, their potential for human medicine and healthcare remains highly promising for common science. However, the challenge of obtaining stable producers that yield higher quantities of these antibiotics continues to be a pertinent concern in modern biotechnology. Although the interest in antibiotics of Pseudomonas bacteria has persisted over the past century, many uncertainties still surround the regulation of the biosynthetic pathways of these compounds. Thus, the present review comprehensively studies the genetic organization and regulation of the biosynthesis of these antibiotics and provides a comprehensive summary of the genetic organization of antibiotic biosynthesis pathways in pseudomonas strains, appealing to both molecular biologists and biotechnologists. In addition, attention is also paid to the application of antibiotics in plant protection.

Characterization of a λ-Carrageenase Mutant with the Generation of Long-Chain λ-Neocarrageenan Oligosaccharides

λ-carrageenan oligosaccharides can be widely applied in the food, pharmaceutical, medicine and cosmetic industries due to their abundant bioactivities, and they are important products for the high-value utilization of λ-carrageenan. However, oligosaccharides with different degrees of polymerization have different properties, and the final products of λ-carrageenase reported so far are mainly λ-neocarrabiose, λ-neocarratetraose and λ-neocarrahexaose without longer-chain oligosaccharides. Further research is consequently required. Herein, a mutant λ-carrageenase was constructed by deleting the pyrroloquinoline quinone-like domain of OUC-CglA derived from Maribacter vaceletii. Interestingly, it was discovered that the majority of final products of the mutant OUC-CglA-DPQQ were long-chain oligosaccharides with a polymerization degree of 10-20, which underwent significant changes compared to that of OUC-CglA. Additionally, without the pyrroloquinoline quinone-like domain, fewer inclusion bodies were produced throughout the expression process, and the yield of the λ-carrageenase increased about five-fold. However, compared to its parental enzyme, significant changes were made to its enzymatic properties. Its optimal temperature and pH were 15 °C and pH 7.0, and its specific activity was 51.59 U/mg. The stability of the enzyme decreased. Thus, it was found that the deleting domain was related to the formation of inclusion bodies, the stability of the enzyme, the activity of the enzyme and the composition of the products.

Reduced OxyR positively regulates the prodigiosin biosynthesis in Serratia marcescens FS14

OxyR, a LysR family transcriptional regulator, plays vital roles in bacterial oxidative stress response. In this study, we found that the deletion of oxyR not only inhibited the antioxidant capacity of S. marcescens FS14, but also decreased the production of prodigiosin. Further study revealed that OxyR activated the prodigiosin biosynthesis at the transcriptional level. Complementary results showed that not only the wild-type OxyR but also the reduced form OxyRC199S could activate the prodigiosin biosynthesis. We further demonstrated that reduced form of wild type OxyR could bind to the promoter of pig gene cluster, and identified the binding sites which is different from oxidized OxyR binding sites in E. coli. Our results demonstrated that OxyR in FS14 uses oxidized form to regulate the expression of the antioxidant related genes and utilizes reduced form to activate prodigiosin production. Further in silico analysis suggested that the activation of prodigiosin biosynthesis by reduced OxyR should be general in S. marcesencs. To our knowledge, this is the first report to show that OxyR uses the reduced form to activate the gene's expression, therefore, our results provide a novel regulation mechanism of OxyR.

Chromosomal barcodes for simultaneous tracking of near-isogenic bacterial strains in plant microbiota

DNA-amplicon-based microbiota profiling can estimate species diversity and abundance but cannot resolve genetic differences within individuals of the same species. Here we report the development of modular bacterial tags (MoBacTags) encoding DNA barcodes that enable tracking of near-isogenic bacterial commensals in an array of complex microbiome communities. Chromosomally integrated DNA barcodes are then co-amplified with endogenous marker genes of the community by integrating corresponding primer binding sites into the barcode. We use this approach to assess the contributions of individual bacterial genes to Arabidopsis thaliana root microbiota establishment with synthetic communities that include MoBacTag-labelled strains of Pseudomonas capeferrum. Results show reduced root colonization for certain mutant strains with defects in gluconic-acid-mediated host immunosuppression, which would not be detected with traditional amplicon sequencing. Our work illustrates how MoBacTags can be applied to assess scaling of individual bacterial genetic determinants in the plant microbiota.

PqqF inhibits T6SS secretion by decreasing the pH in Serratia marcescens FS14

Pyrroloquinoline quinone (PQQ) is a redox cofactor with numerous important physiological functions, and the type VI secretion system (T6SS) is commonly found in Gram-negative bacteria and plays important roles in physiological metabolism of the bacteria. In this study, we found that the deletion of pqqF enhanced the secretion of Hcp-1 in Serratia marcesens FS14 in M9 medium. Transcriptional analysis showed that the deletion of pqqF almost had no effect on the expression of T6SS-1. Further study revealed that the increased secretion of Hcp-1 was altered by the pH changes of the culture medium through the reaction catalyzed by the glucose dehydrogenases in FS14. Finally, we demonstrated that decreased pH of culture medium has similar inhibition effects as PQQ induced on the secretion of T6SS-1. This regulation mode on T6SS by pH in FS14 is different from previously reported in other bacteria. Therefore, our results suggest a novel pH regulation mode of T6SS in S. marcesens FS14, and would broaden our knowledge on the regulation of T6SS secretion.

Microbial synthesis of pyrroloquinoline quinone

Pyrroloquinoline quinone (PQQ) is a peptide-modified natural product. PQQ has important physiological functions such as anti-oxidation, anti-aging, and immunity enhancement. However, due to the lack of in-depth understanding of PQQ biosynthesis and regulation, inefficient PQQ production level limits its wide application. Accordingly, there is still an urgent need to develop high-yielding strains for synthesis of PQQ. This paper reviewed the research and development trends on the PQQ biosynthetic pathways, catalytic reaction mechanism of key enzymes, and the selection of high-yielding strains, which also prospects for the future construction of PQQ biosynthetic microbial cell factories.

Multicopy expression of sigma factor RpoH reduces prodigiosin biosynthesis in Serratia marcescens FS14

Regulation of prodigiosin biosynthesis is received wide attention due to the antimicrobial, immunosuppressive and anticancer activities of prodigiosin. Here, we constructed a transposon mutant library in S. marcescens FS14 to identify genes involved in the regulation of prodigiosin biosynthesis. 62 strains with apparently different colors were obtained. Identification of the transposon insertion sites revealed that they are classified into three groups: the coding region of cyaA and two component system eepS/R and the promoter region of rpoH. Since the effect of cyaA and eepS/R genes on prodigiosin was extensively investigated in Serratia marcescens, we chose the mutant of rpoH for further investigation. Further deletion mutation of rpoH gene showed no effect on prodigiosin production suggesting that the effect on prodigiosin production caused by transposon insertion is not due to the deletion of RpoH. We further demonstrated that multicopy expression of RpoH reduced prodigiosin biosynthesis indicating that transposon insertion caused RpoH enhanced expression. Previous results indicate that RpoS is the sigma factor for transcription of pig gene cluster in FS14, to test whether the enhanced expression of RpoH prevents prodigiosin by competing with RpoS, we found that multicopy expression of RpoS could alleviate the prodigiosin production inhibition by enhanced RpoH. We proposed that multicopy expressed RpoH competes with RpoS for core RNA polymerase (RNAP) resulting in decreased transcription of pig gene cluster and prodigiosin production reduction. We also demonstrated that RpoH is not directly involved in prodigiosin biosynthesis. Our results suggest that manipulating the transcription level of sigma factors may be applied to regulate the production of secondary metabolites.

BarA/UvrY differentially regulates prodigiosin biosynthesis and swarming motility in Serratia marcescens FS14

BarA/UvrY, a two-component system and global regulator that controls expression of more than a hundred of genes involved in virulence, motility, biofilm formation, and central carbon metabolism under various stress conditions. In this study, we investigated the function of BarA/UvrY system in Serratia marcescens FS14. The disruption of barA or/and uvrY results in the yield increase of secondary metabolite prodigiosin. We further demonstrated that BarA/UvrY system represses prodigiosin production by inhibiting the transcription level of pig gene cluster with direct binding to the pigA promoter. In addition, deletion of barA or/and uvrY abolished the swarming motility of FS14, but not the swimming motility. We revealed that BarA/UvrY activates swarming through directly upregulating the expression of the biosurfactant synthesis gene swrW rather than flagella system. We also observed that BarA/UvrY positively regulates the resistance to H₂O₂ same as in Escherichia coli highlighting the importance of BarA/UvrY on hydrogen peroxide resistance. Our results demonstrated that the BarA/UvrY system differentially regulates the biosynthesis of the secondary metabolite prodigiosin and swarming motility in S. marcescens FS14. Comparison of our results with those observed for Serratia sp. 39006 suggests that BarA/UvrY's role in regulation of secondary metabolite production is different among Serratia species.

Crystal structures of TTHA1265 and TTHA1264/TTHA1265 complex reveal an intrinsic heterodimeric assembly

Zinc peptidase M16 family members are widely distributed in most prokaryotic and eukaryotic organisms. M16 family has been divided into three subfamilies, M16A, M16B and M16C, based on sequence alignments and subunit connectivity. TTHA1264, an M16B protein found in Thermus thermophiles HB8, possesses an HXXEH motif essential for Zn²⁺ binding and catalytic activity. TTHA1265 is another member of M16B, which lacks the metal-binding motif but with a conserved active-site R/Y pair commonly found in the C-terminal half of M16 enzymes. Sequence analysis showed that two genes coding for TTHA1264 and TTHA1265 assemble into a single operon in the bacterial genome. Here, we report the crystal structure of TTHA1265 and TTHA1264/TTHA1265 complex from T. thermophilus HB8. Interestingly, when TTHA1264 and TTHA1265 are present alone, TTHA1264 forms a monomer, TTHA1265 forms a homodimer, respectively. However, TTHA264 and TTHA1265 assembled into a heterodimeric complex, indicating that they prefer to form heterodimer. Biochemical data further confirmed the heterodimeric assembly indicating intrinsic heterodimeric assembly of TTHA1264 and TTHA1265. This property of TTHA1264 and TTHA1265 is consistent with the characteristics of the M16B family.

Pyrroloquinoline Quinone Chemistry, Biology, and Biosynthesis

The widely distributed, essential redox factor pyrroloquinoline quinone (PQQ, methoxatin) (1) was discovered in the mid-1960s. The breadth and depth of its biological effects are steadily being revealed, and understanding its biosynthesis at the genomic level is a continuing process. In this review, aspects of the chemistry, biology, biosynthesis, and commercial production of 1 at the gene level, and some applications, are presented from discovery through to mid-2021.

Two component system CpxR/A regulates the prodigiosin biosynthesis by negative control in Serratia marcescens FS14

Prodigiosin is a tripyrrole red secondary metabolite synthesized by many microorganisms, including Serratia marcescens. In this study, we found that the deletion of the gene of sensor kinase CpxA dramatically decreased the prodigiosin production, while the deletion of the gene of the response regulator CpxR or both genes of CpxRA has no effect on prodigiosin production, the kinase function of CpxA is not essential for its regulation on prodigiosin production while the phosphorylation site of CpxR is required. We further demonstrated that the CpxA regulates the prodigiosin biosynthesis at the transcriptional level and the phosphatase activity of CpxA plays vital roles in the regulation of prodigiosin biosynthesis. Finally, we proposed that CpxR/A regulates the prodigiosin biosynthesis by negative control and the phosphorylation level of CpxR may determine the positive or negative control of the genes it regulated.

New developments in RiPP discovery, enzymology and engineering

Covering: up to June 2020Ribosomally-synthesized and post-translationally modified peptides (RiPPs) are a large group of natural products. A community-driven review in 2013 described the emerging commonalities in the biosynthesis of RiPPs and the opportunities they offered for bioengineering and genome mining. Since then, the field has seen tremendous advances in understanding of the mechanisms by which nature assembles these compounds, in engineering their biosynthetic machinery for a wide range of applications, and in the discovery of entirely new RiPP families using bioinformatic tools developed specifically for this compound class. The First International Conference on RiPPs was held in 2019, and the meeting participants assembled the current review describing new developments since 2013. The review discusses the new classes of RiPPs that have been discovered, the advances in our understanding of the installation of both primary and secondary post-translational modifications, and the mechanisms by which the enzymes recognize the leader peptides in their substrates. In addition, genome mining tools used for RiPP discovery are discussed as well as various strategies for RiPP engineering. An outlook section presents directions for future research.

Understanding the biological role of PqqB in Pseudomonas stutzeri using molecular dynamics simulation approach

Phosphate solubilization is an important and widely studied plant growth promoting trait exhibited by many bacteria. Pyrroloquinoline quinone (PQQ), a redox cofactor of methanol and glucose dehydrogenases has been well established as essential for phosphate solubilization. PQQ operon has been well studied in growth promoting rhizobacteria like Pseudomonas spp., Gluconobacter oxydans, Klebsiella pneumoniae, etc. However, the role of PqqB is quite ambiguous as its functional role has been contradicted in many studies. In the present study, we selected Pseudomonas stutzeri - a well-known P solubilizing bacterium as a representative species of the Pseudomonas genus on the basis of phylogenetic and statistical analyses of PqqB proteins. A 3 D model was generated for this protein. Docking of PqqB with PQQ showed good interaction with a theoretical binding affinity of -7.4 kcal/mol. On the other hand, docking of PqqC with 3a-(2-amino-2-carboxy-ethyl)-4,5-dioxo-4,5,6,7,8,9-hexahydro-quinoline-7,9-dicarboxylic acid (AHQQ, immediate precursor of PQQ) showed strong interaction (-10.4 kcal/mol) but the same was low with PQQ (-6.4 kcal/mol). Molecular dynamic simulation of both the complexes showed stable conformation. The binding energy of PqqB-PQQ complex (-182.710 ± 16.585 kJ/mol) was greater than PqqC-PQQ complex (-166.114 ± 12.027 kJ/mol). The results clearly indicated that kinetically there is a possibility that after cyclization of AHQQ to PQQ by PqqC, PQQ can be taken up by PqqB and transported to periplasm for the oxidation of glucose. To the best of our knowledge, this is the first attempt to understand the biological role of PqqB on the basis of molecular interactions and dynamics.Communicated by Ramaswamy H. Sarma.

Cell envelope proteases and peptidases of Pseudomonas aeruginosa: multiple roles, multiple mechanisms

Pseudomonas aeruginosa is a Gram-negative bacterium that is commonly isolated from damp environments. It is also a major opportunistic pathogen, causing a wide range of problematic infections. The cell envelope of P. aeruginosa, comprising the cytoplasmic membrane, periplasmic space, peptidoglycan layer and outer membrane, is critical to the bacteria's ability to adapt and thrive in a wide range of environments. Over 40 proteases and peptidases are located in the P. aeruginosa cell envelope. These enzymes play many crucial roles. They are required for protein secretion out of the cytoplasm to the periplasm, outer membrane, cell surface or the environment; for protein quality control and removal of misfolded proteins; for controlling gene expression, allowing adaptation to environmental changes; for modification and remodelling of peptidoglycan; and for metabolism of small molecules. The key roles of cell envelope proteases in ensuring normal cell functioning have prompted the development of inhibitors targeting some of these enzymes as potential new anti-Pseudomonas therapies. In this review, we summarise the current state of knowledge across the breadth of P. aeruginosa cell envelope proteases and peptidases, with an emphasis on recent findings, and highlight likely future directions in their study.

Bioconversion of recombinantly produced precursor peptide pqqA into pyrroloquinoline quinone (PQQ) using a cell-free in vitro system

Pyrroloquinoline quinone (PQQ) has been recognized as the third class of redox cofactors in addition to the well-known nicotinamides (NAD(P)⁺) and flavins (FAD, FMN). It plays important physiological roles in various organisms and has strong antioxidant properties. The biosynthetic pathway of PQQ involves a gene cluster composed of 4-7 genes, named pqqA-G, among which pqqA is a key gene for PQQ synthesis, encoding the precursor peptide PqqA. To produce recombinant PqqA in E. coli, fusion tags were used to increase the stability and solubility of the peptide, as well simplify the scale-up of the fermentation process. In this paper, pqqA from Gluconobacter oxydans 621H was expressed in E. coli BL21 (DE3) as a fusion protein with SUMO and purified using a hexahistidine (His6) tag. The SUMO fusion protein and His6 tag were specifically recognized and cleaved by the SUMO specific ULP protease, and immobilized-metal affinity chromatography was used to obtain high-purity precursor peptide PqqA. Expression and purification of target proteins was confirmed by Tricine-SDS-PAGE. Finally, the synthesis of PQQ in a cell-free enzymatic reaction in vitro was confirmed by LC-MS.

Biological pincer complexes

At least two types of pincer complexes are known to exist in biology. A metal-pyrroloquinolone quinone (PQQ) cofactor was first identified in bacterial methanol dehydrogenase, and later also found in selected short-chain alcohol dehydrogenases of other microorganisms. The PQQ-associated metal can be calcium, magnesium, or a rare earth element depending on the enzyme sequence. Synthesis of this organic ligand requires a series of accessory proteins acting on a small peptide, PqqA. Binding of metal to PQQ yields an ONO-type pincer complex. More recently, a nickel-pincer nucleotide (NPN) cofactor was discovered in lactate racemase, LarA. This cofactor derives from nicotinic acid adenine dinucleotide via action of a carboxylase/hydrolase, sulfur transferase, and nickel insertase, resulting in an SCS-type pincer complex. The NPN cofactor likely occurs in selected other racemases and epimerases of bacteria, archaea, and a few eukaryotes.

Protease-associated import systems are widespread in Gram-negative bacteria

Bacteria have evolved sophisticated uptake machineries in order to obtain the nutrients required for growth. Gram-negative plant pathogens of the genus Pectobacterium obtain iron from the protein ferredoxin, which is produced by their plant hosts. This iron-piracy is mediated by the ferredoxin uptake system (Fus), a gene cluster encoding proteins that transport ferredoxin into the bacterial cell and process it proteolytically. In this work we show that gene clusters related to the Fus are widespread in bacterial species. Through structural and biochemical characterisation of the distantly related Fus homologues YddB and PqqL from Escherichia coli, we show that these proteins are analogous to components of the Fus from Pectobacterium. The membrane protein YddB shares common structural features with the outer membrane ferredoxin transporter FusA, including a large extracellular substrate binding site. PqqL is an active protease with an analogous periplasmic localisation and iron-dependent expression to the ferredoxin processing protease FusC. Structural analysis demonstrates that PqqL and FusC share specific features that distinguish them from other members of the M16 protease family. Taken together, these data provide evidence that protease associated import systems analogous to the Fus are widespread in Gram-negative bacteria.

To grow and cause infection bacteria must obtain essential nutrients from their environment or host. The element iron is one such nutrient and is often contained inside proteins, the building blocks of hosts cells. Bacteria that cause disease in plants are able to extract iron from plant proteins, by importing the protein and cutting it up once inside the bacterial cell. While it was known that specific bacteria that infect plants can do this, it was unclear if other bacteria that infect humans and animals are also able to import host proteins. In this work we analysed the genetic sequences of bacteria and found that genes responsible for importing and processing proteins are widespread in bacteria that cause disease in humans, animals and plants. We analysed the structure and chemistry of the protein products of these genes and found that they possess characteristics that are necessary and sufficient for importing and processing proteins. Our conclusion from this work is that the ability to import host proteins to gain nutrients is common in bacteria.

A two-component protease in Methylorubrum extorquens with high activity toward the peptide precursor of the redox cofactor pyrroloquinoline quinone

Pyrroloquinoline quinone is a prominent redox cofactor in many prokaryotes, produced from a ribosomally synthesized and post-translationally modified peptide PqqA via a pathway comprising four conserved proteins PqqB-E. These four proteins are now fairly well-characterized and span radical SAM activity (PqqE), aided by a peptide chaperone (PqqD), a dual hydroxylase (PqqB), and an eight-electron, eight-proton oxidase (PqqC). A full description of this pathway has been hampered by a lack of information regarding a protease/peptidase required for the excision of an early, cross-linked di-amino acid precursor to pyrroloquinoline quinone. Herein, we isolated and characterized a two-component heterodimer protein from the α-proteobacterium Methylobacterium (Methylorubrum) extorquens that can rapidly catalyze cleavage of PqqA into smaller peptides. Using pulldown assays, surface plasmon resonance, and isothermal calorimetry, we demonstrated the formation of a complex PqqF/PqqG, with a K_D of 300 nm We created a molecular model of the heterodimer by comparison with the Sphingomonas sp. A1 M16B Sph2681/Sph2682 protease. Analysis of time-dependent patterns for the appearance of proteolysis products indicates high specificity of PqqF/PqqG for serine side chains. We hypothesize that PqqF/PqqG initially cleaves between the PqqE/PqqD-generated cross-linked form of PqqA, with nonspecific cellular proteases completing the release of a suitable substrate for the downstream enzyme PqqB. The finding of a protease that specifically targets serine side chains is rare, and we propose that this activity may be useful in proteomic analyses of the large family of proteins that have undergone post-translational phosphorylation at serine.

Occurrence, function, and biosynthesis of mycofactocin

Mycofactocin is a member of the rapidly growing class of ribosomally synthesized and post-translationally modified peptide (RiPP) natural products. Although the mycofactocin biosynthetic pathway is widely distributed among Mycobacterial species, the structure, function, and biosynthesis of the pathway product remain unknown. This mini-review will discuss the current state of knowledge regarding the mycofactocin biosynthetic pathway. In particular, we focus on the architecture and distribution of the mycofactocin biosynthetic cluster, mftABCDEF, among the Actinobacteria phylum. We discuss the potential molecular and physiological role of mycofactocin. We review known biosynthetic steps involving MftA, MftB, MftC, and MftE and relate them to pyrroloquinoline quinone biosynthesis. Lastly, we propose the function of the remaining putative biosynthetic enzymes, MftD and MftF.

Crystal structure of MBP-PigG fusion protein and the essential function of PigG in the prodigiosin biosynthetic pathway in Serratia marcescens FS14

Prodigiosin, a tripyrrole red pigment is synthesized by Serratia and some other microbes through a bifurcated biosynthesis pathway; MBC (4-methoxy-2,2'-bipyrrole-5-carbaldehyde) and MAP (2-methyl-3-n-amyl-pyrrole) are synthesized separately and then condensed by PigC to form prodigiosin. PigI, PigG and PigA have been shown to be involved in the first steps of MBC biosynthesis (proline incorporation). The crystal structure of PigG was resolved to elucidate its function and mechanism. PigG, an acyl carrier protein (ACP), features the ACP architecture:, a helical bundle fold containing three major helices and a minor distorted helix together with a conserved "S" motif. An in-frame deletion mutation of the pigG gene abolished the synthesis of prodigiosin in Serratia marcescens FS14. The production of prodigiosin was fully restored by complementation of intact pigG; however the S36A mutant was not able to restore function in the in-frame deletion pigG mutant, indicating that PigG and the conserved serine residue (S36) of PigG are essential for the synthesis of prodigiosin.