Cloning and Molecular Characterization of the phlD Gene Involved in the Biosynthesis of “Phloroglucinol”, a Compound with Antibiotic Properties from Plant Growth Promoting Bacteria Pseudomonas spp.

phlD is a novel kind of polyketide synthase involved in the biosynthesis of non-volatile metabolite phloroglucinol by iteratively condensing and cyclizing three molecules of malonyl-CoA as substrate. Phloroglucinol or 2,4-diacetylphloroglucinol (DAPG) is an ecologically important rhizospheric antibiotic produced by pseudomonads; it exhibits broad spectrum anti-bacterial and anti-fungal properties, leading to disease suppression in the rhizosphere. Additionally, DAPG triggers systemic resistance in plants, stimulates root exudation, as well as induces phyto-enhancing activities in other rhizobacteria. Here, we report the cloning and analysis of the phlD gene from soil-borne gram-negative bacteria—Pseudomonas. The full-length phlD gene (from 1078 nucleotides) was successfully cloned and the structural details of the PHLD protein were analyzed in-depth via a three-dimensional topology and a refined three-dimensional model for the PHLD protein was predicted. Additionally, the stereochemical properties of the PHLD protein were analyzed by the Ramachandran plot, based on which, 94.3% of residues fell in the favored region and 5.7% in the allowed region. The generated model was validated by secondary structure prediction using PDBsum. The present study aimed to clone and characterize the DAPG-producing phlD gene to be deployed in the development of broad-spectrum biopesticides for the biocontrol of rhizospheric pathogens.


Introduction
Plant growth-promoting rhizobacteria (PGPR) are bacteria that colonize some or all parts of the rhizosphere environment and have the capability to promote plant growth [1][2][3] either directly by antibiosis or indirectly by quorum sensing. PGPR produce non-volatile metabolites that can directly stimulate plant growth, inhibit plant pathogens, and/or induce host-defense mechanisms against pathogens [4,5]. Pseudomonas fluorescens is an important group of PGPR that suppress root and seedling diseases by producing nonvolatile secondary metabolite phloroglucinol. Genetic methods [6][7][8] and direct isolation from the soils of diseased plants [9][10][11] have shown the importance of DAPG and its derivatives as biocontrol activity agents. These compounds act as antibiotics, antimicrobials or antifungals, signaling molecules, and pathogenicity factors. Several antibacterial and antifungal compounds from plants have been characterized [12] and their mechanisms of action have been delineated [13]; biosynthesis and genetic regulation of DAPG in the Pseudomonas spp. have been the focus of active research. Figure 1. Schematic representation of a ~6.5 kb genomic fragment of Pseudomonas harboring the genes responsible for the biosynthesis of 2,4-DAPG by phl operon. phl operon comprises four genes phlA, phlB, phlC, and phlD. The operon is flanked on either side by phlE and phlF genes that are separately transcribed and coded for the putative efflux and regulatory (repressor) proteins, respectively. They are not required for phloroglucinol production. The DAPG gene cluster is self-sufficient for the biosynthesis and regulation of 2,4diacetylphloroglucinol. Amongst all genes, phlD is the key gene responsible for the production of (MAPG), while, phlA, phlB, and phlC are necessary to convert MAPG to 2,4-DAPG. Products of these genes resemble neither type I nor type II PKS enzyme systems. Rather, PhlD shows similarity to plant chalcone synthases, indicating that phloroglucinol synthesis is mediated by a novel kind of PKS [17][18][19].
Apart from changes in gene expression, the production of 2,4-DAPG in many strains of fluorescent Pseudomonas spp. is stimulated by physical factors, such as a concentration of glucose [20] or concentrations of sucrose/ethanol [19,21]. Moreover, zinc sulfate and ammonium molybdate have been reported to favor 2,4-DAPG production in some strains, whereas inorganic phosphate in general has an inhibitory effect [20].
Our laboratory has isolated and cloned and characterized phlA, phlB, and phlC; the downstream genes [26][27][28] of phl operon, and the current study focuses on the cloning and characterization of the phlD gene, an upstream/first committed step of the phl operon of Pseudomonas and its in-depth characterization to obtain deep insight into the PHLD function. This will help in fine-tuning (upregulating/downregulating) the bio-synthesis of 2,4-DAPG in response to potent fungal and bacterial pathogens for improving the biocontrols of plant pathogens.

Genomic DNA Isolation and PCR Amplification of the phlD Gene
Genomic DNA of the Pseudomonas spp. strain RS9 (KP057506) [29] was isolated as described earlier [26]. The phlD gene from Pseudomonas spp. was amplified by the polymerase chain reaction-based strategy. The forward and reverse primers were designed using the PRIMER 3 tool, viz, phlD (FP): CCGACTAGTAGGACTTGTCATGTC- Figure 1. Schematic representation of a~6.5 kb genomic fragment of Pseudomonas harboring the genes responsible for the biosynthesis of 2,4-DAPG by phl operon. phl operon comprises four genes phlA, phlB, phlC, and phlD. The operon is flanked on either side by phlE and phlF genes that are separately transcribed and coded for the putative efflux and regulatory (repressor) proteins, respectively. They are not required for phloroglucinol production. The DAPG gene cluster is self-sufficient for the biosynthesis and regulation of 2,4diacetylphloroglucinol. Amongst all genes, phlD is the key gene responsible for the production of (MAPG), while, phlA, phlB, and phlC are necessary to convert MAPG to 2,4-DAPG. Products of these genes resemble neither type I nor type II PKS enzyme systems. Rather, PhlD shows similarity to plant chalcone synthases, indicating that phloroglucinol synthesis is mediated by a novel kind of PKS [17][18][19].
Apart from changes in gene expression, the production of 2,4-DAPG in many strains of fluorescent Pseudomonas spp. is stimulated by physical factors, such as a concentration of glucose [20] or concentrations of sucrose/ethanol [19,21]. Moreover, zinc sulfate and ammonium molybdate have been reported to favor 2,4-DAPG production in some strains, whereas inorganic phosphate in general has an inhibitory effect [20].
Our laboratory has isolated and cloned and characterized phlA, phlB, and phlC; the downstream genes [26][27][28] of phl operon, and the current study focuses on the cloning and characterization of the phlD gene, an upstream/first committed step of the phl operon of Pseudomonas and its in-depth characterization to obtain deep insight into the PHLD function. This will help in fine-tuning (upregulating/downregulating) the bio-synthesis of 2,4-DAPG in response to potent fungal and bacterial pathogens for improving the biocontrols of plant pathogens.

Genomic DNA Isolation and PCR Amplification of the phlD Gene
Genomic DNA of the Pseudomonas spp. strain RS9 (KP057506) [29] was isolated as described earlier [26]. The phlD gene from Pseudomonas spp. was amplified by the polymerase chain reaction-based strategy. The forward and reverse primers were designed using the PRIMER 3 tool, viz, phlD (FP): CCGACTAGTAGGACTTGTCATGTCTACTCTTTG and phlD (RP): GGAAAGCTTCGTGCAATGTGTTGGTCTGTCA were designed using the nucleotide sequence of Pseudomonas fluorescens (U41818) available in the EMBL database. Restriction sites for the enzymes SpeI and HindIII were incorporated at the 5 ends of forward and reverse primers (underlined sequences), respectively. The PCR reaction mixture consisted of 10 pmol of each primer, 50 ng of template DNA, 50 mM KCl, 10 mM Tris-HCl (pH 9.0), 0.1% Triton, 2.5 mM MgCl 2 , 0.2 mM of each dNTP, and a 1.25 unit of Phusion Taq DNA polymerase in 100 µL of volume. The thermal cycling was performed with an initial denaturation cycle of 3 min at 98 • C, followed by 30 cycles of (i) denaturation at 98 • C for 20 s; (ii) annealing for 30 s at 55 • C; (iii) extension for 30 s at 72 • C, as well as one cycle of the final extension for 7 min at 72 • C.

Cloning of the phlD Gene in pBluescript (SK+) Vector
For cloning of the PCR amplified phlD gene into the pBluescript (SK+) vector, the PCR product (insert) and pBluescript (SK+) vector DNA were double digested with SpeI and HindIII. The reaction mixture was incubated at 37 • C for 3 hours. Restricted DNA was gelpurified using a Zymo clean gel DNA recovery kit. Purified 50 ng of linearized pBluescript vector and 100 ng of a double-digested PCR product were ligated using T 4 DNA ligase. The ligated mixture was incubated at 4 • C overnight for ligation and used for transformation into E. coli. The transformed colonies (white in color) obtained after overnight incubation at 37 • C were picked and streaked onto fresh LA-carbenicillin (100 µg/mL) plates.

Confirmation of Cloning and Sequencing
Recombinant colonies were confirmed by restriction digestion with SpeI+HindIII enzymes. The restricted DNA samples were analyzed on 1.2% agarose gel. The complete nucleotide sequence was determined by the Sanger di-deoxy sequencing. M13F and M13R primers were used for sequencing. phlD gene-specific primers were also used for confirming the sequence. The final sequence was determined from both strands and a comparison of phlD nucleic acid and amino acid sequences with already existing sequences was performed. The deduced amino acid sequence of PHLD from Pseudomonas RS-9 was compared with type III PKS from gram-positive bacteria and other plants by a multiple sequence alignment using MAFFT version 7.271 program [30] with the L-INS-I strategy and output in Phylip format. A similarity score for each nucleotide of the aligned sequences was calculated by ESPRIPT 3.0 [31] (https://espript.ibcp.fr/ESPript/ESPript/, accessed on 20 February 2022) with default parameters. Conserved domain annotation analysis was performed using InterProScan [32].

Phylogenetic Analysis
For estimation of the phylogenetic relationship between PHLD from various Pseudomonas strains and type III PKS from gram-positive bacteria and plants, the amino acid sequences were retrieved from the NCBI database. A multiple sequence alignment for the respective amino acid sequences was performed by Clustal Omega [33] and an un-rooted tree was constructed in MEGA10 [34] using the maximum likelihood (ML) method. Tree topology was searched using the nearest neighbor interchanges (NNIs) algorithm [35]. The LG+G+I substitution model was employed. The gamma shape parameter was estimated directly from the data and the analysis was performed using 1000 bootstrap replicates. The proportion of invariable sites was fixed. The tree was obtained in the Newick format.

Structure Prediction
The model of the PHLD protein was predicted using the I-TASSER server (http: //zhanglab.ccmb.med.umich.edu/I-TASSER/, accessed on 25 February 2022) [36]. I-TASSER (Iterative Threading ASSEmbly Refinement) is a hierarchical approach to protein structures and function prediction. Structural templates were first identified from PDB by the multiple threading approach, LOMETS; full-length atomic models were then constructed by iterative template fragment assembly simulations. The generated model was refined using ModRefiner (http://zhanglab.ccmb.med.umich.edu/ModRefiner/, accessed on 3 March 2022). ModRefiner is an algorithm for high-resolution protein structure refinement. Both side-chain and backbone atoms were completely flexible during structure refinement simulations. ModRefiner allowed the assignment of a second structure that was Antibiotics 2023, 12, 260 4 of 16 used as a reference to which the refinement simulations were driven. The ModRefiner was used to draw the initial starting model of PHLD closer to its native state.

Ramachandran Plot Analysis
The stereochemical properties of the PHLD protein were assessed by the Ramachandran plot analysis using RAMPAGE [37]. This allowed visualization of energetically allowed regions for backbone dihedral angles ψ against ϕ of amino acid residues in the PHLD protein structure. The residues in the disallowed region were further refined by using Modloop (https://modbase.compbio.ucsf.edu/modloop/, accessed on 5 March 2022). Modloop relies on MODELLER, which predicts the loop conformations of PHLD by the satisfaction of spatial restraints, without relying on a database of known protein structures [38].

Validation and Visualization of Modeled Structure
The validation of the modeled structure was performed using PDBsum [39] and PROCHECK [40]. Structure visualization was performed using PyMOL. The predicted model of the protein was submitted to the Protein Model Database [41] (http://srv00.recas. ba.infn.it/PMDB/main.php, accessed on 10 March 2022).

Results
PCR amplification of the phlD gene from the genomic DNA of the Pseudomonas spp. strain RS9 (KP057506) [29] resulted in a fragment of 1 kb ( Figure 2). The amplified PCR product and pBluescript control vector were then restricted with SpeI and HindIII restriction enzymes. This resulted in a 1 kb fragment PCR product with sticky ends (insert) and 3 kb of linearized control vector pBluescript (SK+) with sticky ends for SpeI and HindIII ( Figure 3). The purified double-digested PCR product (insert) was ligated into the linearized pBluescript vector. lowed regions for backbone dihedral angles ψ against φ of PHLD protein structure. The residues in the disallowed regi using Modloop (https://modbase.compbio.ucsf.edu/modloop/ Modloop relies on MODELLER, which predicts the loop con satisfaction of spatial restraints, without relying on a database [38].

Validation and Visualization of Modeled Structure
The validation of the modeled structure was performe PROCHECK [40]. Structure visualization was performed us model of the protein was submitted to the Protein Model D cas.ba.infn.it/PMDB/main.php, accessed on 10 March 2022).

Results
PCR amplification of the phlD gene from the genomic DN strain RS9 (KP057506) [29] resulted in a fragment of 1 kb (Fi product and pBluescript control vector were then restricted striction enzymes. This resulted in a 1 kb fragment PCR prod and 3 kb of linearized control vector pBluescript (SK+) wit HindIII ( Figure 3). The purified double-digested PCR product linearized pBluescript vector.    The ligated mixture was transformed into E. coli DH-5α competent cells and five randomly picked white colonies were used for the plasmid isolation. The presence of the phlD gene was confirmed by restriction digestion with SpeI+HindIII enzymes that released the expected fragment of ~1 kb ( Figure 4).  1.078 kb fragment of purified PCR product. The PCR product was digested with SpeI+HindIII.
The ligated mixture was transformed into E. coli DH-5α competent cells and five randomly picked white colonies were used for the plasmid isolation. The presence of the phlD gene was confirmed by restriction digestion with SpeI+HindIII enzymes that released the expected fragment of~1 kb ( Figure 4).  The ligated mixture was transformed into E. coli DH-5α competent cells and five randomly picked white colonies were used for the plasmid isolation. The presence of the phlD gene was confirmed by restriction digestion with SpeI+HindIII enzymes that released the expected fragment of ~1 kb ( Figure 4).  Sanger sequencing of the cloned phlD was carried to check its identity and the results revealed that it consisted of 1078 nucleotides with an open reading frame of 1050 bp. Based on the blast results, the phlD gene was found to be of full-length coding for 349 amino acids. The cloned phlD gene showed considerable homology with the other known genes, indicating a common descent. The deduced amino acid sequence of 349 amino acids (~38.3 KDa) showed significant similarity with the homologs of PHLD (Figures 5 and 6).  The deduced amino acid sequence from Pseudomonas RS-9 was compared with type III PKSs from Gram-positive bacteria and CHS/STS from plants. The functional roles of key amino acid residues found in type III PKSs/CHS/STS were found in PHLD proteins and other bacterial-type III PKSs (Figure 7), such as plant C169 (cysteine-169), responsible for the catalytic activities of plant CHSs, S158 in plants (serine-158), and Q166 in plants (glutamine-166), and were conserved in PHLD proteins. C135 (cysteine-135) and C195 (cysteine-195) played roles in substrate specificity and K180 (lysine-180), which are important for the enzymatic structure and function in plant-type III PKSs, and are replaced in the bacterial counterparts. Threonine, serine, and asparagine, respectively, replaced these amino acids in the PHLD sequences (Figure 7). The deduced amino acid sequence from Pseudomonas RS-9 was compared with type III PKSs from gram-positive bacteria and CHS/STS from plants. The functional roles of key amino acid residues found in type III PKSs/CHS/STS were found in PHLD proteins and other bacterial-type III PKSs (Figure 7), such as plant C169 (cysteine-169), responsible for the catalytic activities of plant CHSs, S158 in plants (serine-158), and Q166 in plants (glutamine-166), and were conserved in PHLD proteins. C135 (cysteine-135) and C195 (cysteine-195) played roles in substrate specificity and K180 (lysine-180), which are important for the enzymatic structure and function in plant-type III PKSs, and are replaced in the bacterial counterparts. Threonine, serine, and asparagine, respectively, replaced these amino acids in the PHLD sequences (Figure 7).  The deduced amino acid sequence from Pseudomonas RS-9 was compared with type III PKSs from Gram-positive bacteria and CHS/STS from plants. The functional roles of key amino acid residues found in type III PKSs/CHS/STS were found in PHLD proteins and other bacterial-type III PKSs (Figure 7), such as plant C169 (cysteine-169), responsible for the catalytic activities of plant CHSs, S158 in plants (serine-158), and Q166 in plants (glutamine-166), and were conserved in PHLD proteins. C135 (cysteine-135) and C195 (cysteine-195) played roles in substrate specificity and K180 (lysine-180), which are important for the enzymatic structure and function in plant-type III PKSs, and are replaced in the bacterial counterparts. Threonine, serine, and asparagine, respectively, replaced these amino acids in the PHLD sequences (Figure 7).  Analyses of the conserved domains and annotations were performed using Inter-ProScan. The results revealed the presence of two InterPro domains viz. the chalcone/ stilbene_synthases_N-terminal domain (37-200 a.a., IPR001099) and the chalcone/stilbene_ synthases_C-terminal domain (213-344 a.a., IPR012328). These two domains belong to the Polyketide_synthase_type III InterPro family (IPR011141). A maximum likelihood (ML) tree was constructed to compare the phylogenetic relation of PHLD from Pseudomonas to Type III PKSs from bacteria and plants (PKS from other gram-positive bacteria and CHS/STS from plants). Upon comparison, eight PHLD sequences clustered into a separate group along with PKS from Streptomyces griseus (PKS). PKS from gram-positive bacteria and CHS/STS from plants also clustered in a separate group in the ML tree ( Figure 8).

8
(Q166) residue conserved in most plant CHSs and corresponding to Q166 in the PKS of Streptomyces griseus and Q135 in PHLD proteins. (4) The glycine cysteine (GC) box corresponds to the conserved region with its catalytic cysteine residue. (5) Lysine (K180) residue, which corresponds to asparagine (N149) in PHLD, conserved strictly in the plant. (6) C195 involved in the product specificity in the plant CHS.

Discussion
Special attention has been given to the antibiotic-producing fluorescent species of Pseudomonas due to their antibacterial [10,46], antifungal [47][48][49][50], and antiviral [51] abilities to control a wide variety of plant diseases. Advances in molecular techniques have also improved our potential to study the DAPG-producing antibacterial strains for their mechanisms of pathogen suppression and growth promotion. Breakthroughs in genomics [52][53][54][55] and transgenic [56,57] research to impart biotic/abiotic tolerance [58][59][60] or engineer traits in crops [61,62] have also driven the research in the field of biocontrol using DAPG producing strains. Genetic engineering approaches have been employed for the high-level production of phloroglucinol. Since the genetic background and metabolism of Pseudomonas have not been elucidated completely and the host does not respond well to genetic manipulation, the heterologous expression of phlD in E. coli is a great approach for increasing the accumulation of phloroglucinol in cultures [63][64][65].
DAPG is known to have antifungal properties and is produced by tandem activities of six genes viz. phlA, phlB, phlC, phlD, phlE, and phlF. These genes are organized as an operon onto a single nucleotide fragment of size~6.5 kb. Among these six genes, phlD alone is important for the synthesis of MAPG. Although phlA, phlB, and phlC are also required for the synthesis of MAPG, phlD is the most essential. It has been proved that MAPG is synthesized only in the presence of phlD and in its absence, the cells converted exogenous MAPG to 2,4-DAPG but were unable to produce either compound themselves. This attribute makes phlD an important and useful marker of the genetic diversity and population structure among the 2,4-DAPG producers [14]. Thus, probes and primers specific for phlD have been used in combination with colony hybridization and polymerase chain reaction (PCR) to quantify the population sizes of 2,4-DAPG producers in the rhizosphere [11,66,67].
phlD shows a remarkable similarity to CHS/STS enzymes from plants. This is surprising because most of the microbial antibiotic enzymes are known to be synthesized via type I or type II PKSs [45,[68][69][70][71]. Structural similarities between phlD and CHS/STS enzymes point to the common evolutionary descent and similarities in the roles they play during plant defense strongly support the instances of gene exchange between plants and bacteria [46,72]. The absence of the acyl carrier protein gene from the phl operon further confirms the similarity with the CHS/STS gene family. Pseudomonas spp. strain RS-9 was used for the isolation and cloning of the full-length phlD gene. The primers were designed using the Pseudomonas fluoresens (U41818) phlD gene sequence as a reference and Pseudomonas spp. strain RS-9 as the template. To amplify the full-length phl gene, we first standardized the PCR conditions. A gradient PCR was set in a temperature range of 50 • C to 60 • C to optimize the Tm for the reaction. At a lower Tm, multiple bands were obtained and at a very high Tm, faint amplification was obtained. The optimum amplification of~1 kb for the phlD gene was obtained at 55 • C. This amplicon was restricted, purified, and ligated to the pBluescript vector and transformed into E. coli cells.
The cloned phlD gene through our investigation was confirmed by Sanger sequencing and it consisted of 1078 nucleotides with an open reading frame from 10 to 1059. The longest ORF of the phlD gene was found to be 1050 bp. Based on the blast results, the cloned phlD gene was found to be full-length, coding for 349 amino acids. This is consistent with the other reports [73] (Figure 5) on the length of amino acid coding phl genes. The cloned phlD gene showed 93% homology with phlD genes from different Pseudomonas strains, such as Pseudomonas sp. Q12-87, Pseudomonas sp. K96.27, Pseudomonas sp. PITR2, Pseudomonas sp. Q37-87, Pseudomonas sp. 12. The deduced amino acid sequence of 349 amino acids (~38.3 KDa) showed 97% similarity with the homologs of PHLD [73]. The phlD gene is of utmost importance to the DAPG gene cluster as MAPG synthesis does not occur without it.
The deduced amino acid sequence of the cloned phlD gene consisting of 349 amino acids was aligned pairwise with the Pseudomonas fluorescens (U41818) PHLD protein. The alignment revealed that there were few mutations in the protein sequence of the cloned gene. These mutations were authentic as we amplified and cloned the gene using highfidelity Phusion Taq polymerase. Since this Taq polymerase has 3 of proofreading activity, the chances of mis-amplifying or incorporating wrong bases are meager. These mutations need further characterization by site-directed mutagenesis.
A comparison of the deduced amino acid sequence from Pseudomonas RS-9, type III PKS from plants (CHS/STS), and other gram-positive bacteria indicated that PHLD and plant CHSs displayed common features. Comparison of the active site region indicated replacements of C135, C195, and K180 with threonine, serine, and asparagine that might have influenced their substrate specificities. Lysine and asparagine codons differ only at the third nucleotide position, and a single transversion can yield an asparagine instead of a lysine. The cluster analysis clearly distinguished between PhlD and plant CHS/STS by clustering them into separate groups [74][75][76]. Type III PKS from gram-positive bacteria clustered between CHS/STS from plants and PHLD from eight Pseudomonas strains. PKS closest to PHLD was the Streptomyces gresius PKS based on cluster analysis. The possibility that type III PKSs from fluorescent pseudomonads, gram-positive bacteria, and higher plants arose independently and may represent convergent evolution of the key enzymes involved in the biosynthesis of secondary metabolites as speculated earlier [46,73], corroborating out results of the cluster analysis.
The three-dimensional structure of the PHLD protein predicted using the I-TASSER server was based on the template crystal structure of Mycobacterium tuberculosis polyketide synthase 11 (PKS11) (PDB entry 4JAP) [42]. The PHLD model had an excellent C-score of 1.61 indicating a good quality model. C-score ranged from −5 to 2 and this higher value indicates the high quality of the model. Similarly, a TM score >0.5 indicated a model of correct topology, and a TM score <0.17 meant a random similarity. A TM score of 0.94 ± 0.05 for PHLD indicates the precision of the predicted topology. Moreover, >90% residues (94.3% residues) in the favored region of the Ramachandran plot reaffirmed the stereochemical stability of the generated refined molecule.

Conclusions
Pseudomonas strains have been used as potent biocontrol agents for controlling plant diseases because of the production of metabolites with antibiotic properties [77]. Amongst them, fluorescent pseudomonads are suitable for application as biocontrol agents and are best-characterized by biocontrol PGPR [78]. The biocontrol property of Pseudomonas is attributed to the synthesis of phloroglucinol-a secondary metabolite with antibiotic properties-produced by genes encoded by the phl operon. The phlD gene present in the phl operon is singularly involved in the synthesis of MAPG, which is processed into phloroglucinol. Though PhlD exhibits condensing activity on malonyl CoA to produce phloroglucinol; the substrate specificity of the enzyme is not limited to malonyl CoA compared to other type III PKS enzymes. It also catalyzes "C4-C12 aliphatic acyl-CoAs and phenylacetyl-CoA" as substrates to form tri-to heptaketide pyrones [25]. The same is evidenced by the homology modelling of PhlD that reveals the presence of a buried tunnel that protrudes out of the active site to accommodate the binding of acyl-CoAs. Structural details revealed from our findings can be used for targeted mutagenesis and rational designs to successfully alter the substrate specificity of PhlD to produce derivatized products with higher potency for antibiosis. Since, phlD is the first committed step of DAPG biosynthesis, targeting substrate specificity of PhlD would be a prudent way to enhance the biocontrol activities of Pseudomonas spp. that otherwise are present as long-lasting indigenous communities in several agro-ecosystems to augment the capability to protect the plant root system from numerous soil-borne plant diseases. Our results of cloning and structural delineation of phlD will provide novel strategies for combinatorial biosynthesis of natural but pharmaceutically important metabolites with enhanced antibacterial and biocontrol effects.