Linking a Gene Cluster to Atranorin, a Major Cortical Substance of Lichens, through Genetic Dereplication and Heterologous Expression.

The depside and depsidone series compounds of polyketide origin accumulate in the cortical or medullary layers of lichen thalli. Despite the taxonomic and ecological significance of lichen chemistry and its pharmaceutical potentials, there has been no single piece of genetic evidence linking biosynthetic genes to lichen substances. Thus, we systematically analyzed lichen polyketide synthases (PKSs) for categorization and identification of the biosynthetic gene cluster (BGC) involved in depside/depsidone production. Our in-depth analysis of the interspecies PKS diversity in the genus Cladonia and a related Antarctic lichen, Stereocaulon alpinum, identified 45 BGC families, linking lichen PKSs to 15 previously characterized PKSs in nonlichenized fungi. Among these, we identified highly syntenic BGCs found exclusively in lichens producing atranorin (a depside). Heterologous expression of the putative atranorin PKS gene (coined atr1) yielded 4-O-demethylbarbatic acid, found in many lichens as a precursor compound, indicating an intermolecular cross-linking activity of Atr1 for depside formation. Subsequent introductions of tailoring enzymes into the heterologous host yielded atranorin, one of the most common cortical substances of macrolichens. Phylogenetic analysis of fungal PKS revealed that the Atr1 is in a novel PKS clade that included two conserved lichen-specific PKS families likely involved in biosynthesis of depsides and depsidones. Here, we provide a comprehensive catalog of PKS families of the genus Cladonia and functionally characterize a biosynthetic gene cluster from lichens, establishing a cornerstone for studying the genetics and chemical evolution of diverse lichen substances. IMPORTANCE Lichens play significant roles in ecosystem function and comprise about 20% of all known fungi. Polyketide-derived natural products accumulate in the cortical and medullary layers of lichen thalli, some of which play key roles in protection from biotic and abiotic stresses (e.g., herbivore attacks and UV irradiation). To date, however, no single lichen product has been linked to respective biosynthetic genes with genetic evidence. Here, we identified a gene cluster family responsible for biosynthesis of atranorin, a cortical substance found in diverse lichen species, by categorizing lichen polyketide synthase and reconstructing the atranorin biosynthetic pathway in a heterologous host. This study will help elucidate lichen secondary metabolism, harnessing the lichen's chemical diversity, hitherto obscured due to limited genetic information on lichens.

Insights into modular polyketide synthase loops aided by repetitive sequences.

The loops of modular polyketide synthases (PKSs) serve diverse functions but are largely uncharacterized. They frequently contain amino acid repeats resulting from genetic events such as slipped-strand mispairing. Determining the tolerance of loops to amino acid changes would aid in understanding and engineering these multidomain molecule factories. Here, tandem repeats in the DNA encoding 949 modules within 129 cis-acyltransferase PKSs were cataloged, and the locations of the corresponding amino acids within the module were identified. The most frequently inserted interdomain loop corresponds with the updated module boundary immediately downstream of the ketosynthase (KS), while the loops bordering the dehydratase are nearly intolerant to such insertions. From the 949 modules, no repetitive sequence loop insertions are located within ACP, and only 2 reside within KS, indicating the sensitivity of these domains to alteration.

Genome Mining and Evolutionary Analysis Reveal Diverse Type III Polyketide Synthase Pathways in Cyanobacteria.

Cyanobacteria are prolific producers of natural products, including polyketides and hybrid compounds thereof. Type III polyketide synthases (PKSs) are of particular interest, due to their wide substrate specificity and simple reaction mechanism, compared with both type I and type II PKSs. Surprisingly, only two type III PKS products, hierridins, and (7.7)paracyclophanes, have been isolated from cyanobacteria. Here, we report the mining of 517 cyanobacterial genomes for type III PKS biosynthesis gene clusters. Approximately 17% of the genomes analyzed encoded one or more type III PKSs. Together with already characterized type III PKSs, the phylogeny of this group of enzymes was investigated. Our analysis showed that type III PKSs in cyanobacteria evolved into three major lineages, including enzymes associated with 1) (7.7)paracyclophane-like biosynthesis gene clusters, 2) hierridin-like biosynthesis gene clusters, and 3) cytochrome b5 genes. The evolutionary history of these enzymes is complex, with some sequences partitioning primarily according to speciation and others putatively according to their reaction type. Protein modeling showed that cyanobacterial type III PKSs generally have a smaller active site cavity (mean = 109.035 Å3) compared with enzymes from other organisms. The size of the active site did not correlate well with substrate size, however, the "Gatekeeper" amino acid residues within the active site were strongly correlated to enzyme phylogeny. Our study provides unprecedented insight into the distribution, diversity, and molecular evolution of cyanobacterial type III PKSs, which could facilitate the discovery, characterization, and exploitation of novel enzymes, biochemical pathways, and specialized metabolites from this biosynthetically talented clade of microorganisms.

Pyridoxal-5'-phosphate-dependent bifunctional enzyme catalyzed biosynthesis of indolizidine alkaloids in fungi.

Indolizidine alkaloids such as anticancer drugs vinblastine and vincristine are exceptionally attractive due to their widespread occurrence, prominent bioactivity, complex structure, and sophisticated involvement in the chemical defense for the producing organisms. However, the versatility of the indolizidine alkaloid biosynthesis remains incompletely addressed since the knowledge about such biosynthetic machineries is only limited to several representatives. Herein, we describe the biosynthetic gene cluster (BGC) for the biosynthesis of curvulamine, a skeletally unprecedented antibacterial indolizidine alkaloid from Curvularia sp. IFB-Z10. The molecular architecture of curvulamine results from the functional collaboration of a highly reducing polyketide synthase (CuaA), a pyridoxal-5'-phosphate (PLP)-dependent aminotransferase (CuaB), an NADPH-dependent dehydrogenase (CuaC), and a FAD-dependent monooxygenase (CuaD), with its transportation and abundance regulated by a major facilitator superfamily permease (CuaE) and a Zn(II)Cys6 transcription factor (CuaF), respectively. In contrast to expectations, CuaB is bifunctional and capable of catalyzing the Claisen condensation to form a new C-C bond and the α-hydroxylation of the alanine moiety in exposure to dioxygen. Inspired and guided by the distinct function of CuaB, our genome mining effort discovers bipolamines A-I (bipolamine G is more antibacterial than curvulamine), which represent a collection of previously undescribed polyketide alkaloids from a silent BGC in Bipolaris maydis ATCC48331. The work provides insight into nature's arsenal for the indolizidine-coined skeletal formation and adds evidence in support of the functional versatility of PLP-dependent enzymes in fungi.

Genus level analysis of PKS-NRPS and NRPS-PKS hybrids reveals their origin in Aspergilli.

BACKGROUND: Filamentous fungi produce a vast amount of bioactive secondary metabolites (SMs) synthesized by e.g. hybrid polyketide synthase-nonribosomal peptide synthetase enzymes (PKS-NRPS; NRPS-PKS). While their domain structure suggests a common ancestor with other SM proteins, their evolutionary origin and dynamics in fungi are still unclear. Recent rational engineering approaches highlighted the possibility to reassemble hybrids into chimeras - suggesting molecular recombination as diversifying mechanism. RESULTS: Phylogenetic analysis of hybrids in 37 species - spanning 9 sections of Aspergillus and Penicillium chrysogenum - let us describe their dynamics throughout the genus Aspergillus. The tree topology indicates that three groups of PKS-NRPS as well as one group of NRPS-PKS hybrids developed independently from each other. Comparison to other SM genes lead to the conclusion that hybrids in Aspergilli have several PKS ancestors; in contrast, hybrids are monophyletic when compared to available NRPS genes - with the exception of a small group of NRPSs. Our analysis also revealed that certain NRPS-likes are derived from NRPSs, suggesting that the NRPS/NRPS-like relationship is dynamic and proteins can diverge from one function to another. An extended phylogenetic analysis including bacterial and fungal taxa revealed multiple ancestors of hybrids. Homologous hybrids are present in all sections which suggests frequent horizontal gene transfer between genera and a finite number of hybrids in fungi. CONCLUSION: Phylogenetic distances between hybrids provide us with evidence for their evolution: Large inter-group distances indicate multiple independent events leading to the generation of hybrids, while short intra-group distances of hybrids from different taxonomic sections indicate frequent horizontal gene transfer. Our results are further supported by adding bacterial and fungal genera. Presence of related hybrid genes in all Ascomycetes suggests a frequent horizontal gene transfer between genera and a finite diversity of hybrids - also explaining their scarcity. The provided insights into relations of hybrids and other SM genes will serve in rational design of new hybrid enzymes.

Emulating evolutionary processes to morph aureothin-type modular polyketide synthases and associated oxygenases.

Polyketides produced by modular type I polyketide synthases (PKSs) play eminent roles in the development of medicines. Yet, the production of structural analogs by genetic engineering poses a major challenge. We report an evolution-guided morphing of modular PKSs inspired by recombination processes that lead to structural diversity in nature. By deletion and insertion of PKS modules we interconvert the assembly lines for related antibiotic and antifungal agents, aureothin (aur) and neoaureothin (nor) (aka spectinabilin), in both directions. Mutational and functional analyses of the polyketide-tailoring cytochrome P450 monooxygenases, and PKS phylogenies give contradictory clues on potential evolutionary scenarios (generalist-to-specialist enzyme evolution vs. most parsimonious ancestor). The KS-AT linker proves to be well suited as fusion site for both excision and insertion of modules, which supports a model for alternative module boundaries in some PKS systems. This study teaches important lessons on the evolution of PKSs, which may guide future engineering approaches.

In silico approaches illustrate the evolutionary pattern and protein-small molecule interactions of quinolone synthase from Aegle marmelos Correa.

Quinolone synthase from Aegle marmelos (AmQNS) is a Rutacean-specific plant type III polyketide synthase that synthesizes quinolone, acridone, and benzalacetone with therapeutic potential. Simple architecture and broad substrate affinity of AmQNS make it as one of the target enzymes to produce novel structural scaffolds. Another unique feature of AmQNS despite its high similarity to acridone forming type III polyketide synthase from Citrus microcarpa is the variation in the product formation. Hence, to explore the characteristic features of AmQNS, an in-depth sequence and structure-based bioinformatics analyses were performed. Our studies indicated that AmQNS and its nearest homologs have evolved by a series of gene duplication events and strong purifying selection pressure constrains them in the evolutionary process. Additionally, some amino acid alterations were identified in the functionally important region(s), which can contribute to the functional divergence of the enzyme. Prediction of favorable amino acid substitutions will be advantageous in the metabolic engineering of AmQNS for the production of novel compounds. Furthermore, comparative modeling and docking studies were utilized to investigate the structural behavior and small molecule interaction pattern of AmQNS. The observations and results reported here are crucial for advancing our understanding of AmQNS's phylogenetic position, selection pressure, evolvability, interaction pattern and thus providing the foundation for further studies on the structural and reaction mechanism.

The Structural Enzymology of Iterative Aromatic Polyketide Synthases: A Critical Comparison with Fatty Acid Synthases.

Polyketides are a large family of structurally complex natural products including compounds with important bioactivities. Polyketides are biosynthesized by polyketide synthases (PKSs), multienzyme complexes derived evolutionarily from fatty acid synthases (FASs). The focus of this review is to critically compare the properties of FASs with iterative aromatic PKSs, including type II PKSs and fungal type I nonreducing PKSs whose chemical logic is distinct from that of modular PKSs. This review focuses on structural and enzymological studies that reveal both similarities and striking differences between FASs and aromatic PKSs. The potential application of FAS and aromatic PKS structures for bioengineering future drugs and biofuels is highlighted.

Discriminating the reaction types of plant type III polyketide synthases.

MOTIVATION: Functional prediction of paralogs is challenging in bioinformatics because of rapid functional diversification after gene duplication events combined with parallel acquisitions of similar functions by different paralogs. Plant type III polyketide synthases (PKSs), producing various secondary metabolites, represent a paralogous family that has undergone gene duplication and functional alteration. Currently, there is no computational method available for the functional prediction of type III PKSs. RESULTS: We developed a plant type III PKS reaction predictor, pPAP, based on the recently proposed classification of type III PKSs. pPAP combines two kinds of similarity measures: one calculated by profile hidden Markov models (pHMMs) built from functionally and structurally important partial sequence regions, and the other based on mutual information between residue positions. pPAP targets PKSs acting on ring-type starter substrates, and classifies their functions into four reaction types. The pHMM approach discriminated two reaction types with high accuracy (97.5%, 39/40), but its accuracy decreased when discriminating three reaction types (87.8%, 43/49). When combined with a correlation-based approach, all 49 PKSs were correctly discriminated, and pPAP was still highly accurate (91.4%, 64/70) even after adding other reaction types. These results suggest pPAP, which is based on linear discriminant analyses of similarity measures, is effective for plant type III PKS function prediction. AVAILABILITY AND IMPLEMENTATION: pPAP is freely available at ftp://ftp.genome.jp/pub/tools/ppap/. CONTACT: goto@kuicr.kyoto-u.ac.jp. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

Functional and Structural Analysis of Programmed C-Methylation in the Biosynthesis of the Fungal Polyketide Citrinin.

Fungal polyketide synthases (PKSs) are large, multidomain enzymes that biosynthesize a wide range of natural products. A hallmark of these megasynthases is the iterative use of catalytic domains to extend and modify a series of enzyme-bound intermediates. A subset of these iterative PKSs (iPKSs) contains a C-methyltransferase (CMeT) domain that adds one or more S-adenosylmethionine (SAM)-derived methyl groups to the carbon framework. Neither the basis by which only specific positions on the growing intermediate are methylated ("programming") nor the mechanism of methylation are well understood. Domain dissection and reconstitution of PksCT, the fungal non-reducing PKS (NR-PKS) responsible for the first isolable intermediate in citrinin biosynthesis, demonstrates the role of CMeT-catalyzed methylation in precursor elongation and pentaketide formation. The crystal structure of the S-adenosyl-homocysteine (SAH) coproduct-bound PksCT CMeT domain reveals a two-subdomain organization with a novel N-terminal subdomain characteristic of PKS CMeT domains and provides insights into co-factor and ligand recognition.

Type†III Polyketide Synthases: Functional Classification and Phylogenomics.

Polyketide synthases (PKSs) catalyze the sequential condensation of simple acetate units to produce a large class of natural products, including pharmacologically valuable compounds. PKSs are classified into three types on the basis of their domain structures; type III PKSs have the simplest domain structure, although their products have various structures and functions. The sequence-function relationship is fundamental for predicting enzyme functions, but it has not been well investigated in type III PKSs to date. Consequently, the current methods for predicting type III PKS functions are still immature in comparison with those that target type I/II PKSs. In this review we summarize the current functional and phylogenomic knowledge about type III PKSs and propose a new classification of their enzymatic reactions. We also discuss possible directions for the development of better computational tools for functional prediction of type III PKS homologues.

Multifunctional polyketide synthase genes identified by genomic survey of the symbiotic dinoflagellate, Symbiodinium minutum.

BACKGROUND: Dinoflagellates are unicellular marine and freshwater eukaryotes. They possess large nuclear genomes (1.5-245 gigabases) and produce structurally unique and biologically active polyketide secondary metabolites. Although polyketide biosynthesis is well studied in terrestrial and freshwater organisms, only recently have dinoflagellate polyketides been investigated. Transcriptomic analyses have characterized dinoflagellate polyketide synthase genes having single domains. The Genus Symbiodinium, with a comparatively small genome, is a group of major coral symbionts, and the S. minutum nuclear genome has been decoded. RESULTS: The present survey investigated the assembled S. minutum genome and identified 25 candidate polyketide synthase (PKS) genes that encode proteins with mono- and multifunctional domains. Predicted proteins retain functionally important amino acids in the catalytic ketosynthase (KS) domain. Molecular phylogenetic analyses of KS domains form a clade in which S. minutum domains cluster within the protist Type I PKS clade with those of other dinoflagellates and other eukaryotes. Single-domain PKS genes are likely expanded in dinoflagellate lineage. Two PKS genes of bacterial origin are found in the S. minutum genome. Interestingly, the largest enzyme is likely expressed as a hybrid non-ribosomal peptide synthetase-polyketide synthase (NRPS-PKS) assembly of 10,601 amino acids, containing NRPS and PKS modules and a thioesterase (TE) domain. We also found intron-rich genes with the minimal set of catalytic domains needed to produce polyketides. Ketosynthase (KS), acyltransferase (AT), and acyl carrier protein (ACP) along with other optional domains are present. Mapping of transcripts to the genome with the dinoflagellate-specific spliced leader sequence, supports expression of multifunctional PKS genes. Metabolite profiling of cultured S. minutum confirmed production of zooxanthellamide D, a polyhydroxy amide polyketide and other unknown polyketide secondary metabolites. CONCLUSION: This genomic survey demonstrates that S. minutum contains genes with the minimal set of catalytic domains needed to produce polyketides and provides evidence of the modular nature of Type I PKS, unlike monofunctional Type I PKS from other dinoflagellates. In addition, our study suggests that diversification of dinoflagellate PKS genes comprises dinoflagellate-specific PKS genes with single domains, multifunctional PKS genes with KS domains orthologous to those of other protists, and PKS genes of bacterial origin.

Comparison of 10,11-Dehydrocurvularin Polyketide Synthases from Alternaria cinerariae and Aspergillus terreus Highlights Key Structural Motifs.

Iterative type I polyketide synthases (PKSs) from fungi are multifunctional enzymes that use their active sites repeatedly in a highly ordered sequence to assemble complex natural products. A phytotoxic macrolide with anticancer properties, 10,11-dehydrocurvularin (DHC), is produced by cooperation of a highly reducing (HR) iterative PKS and a non-reducing (NR) iterative PKS. We have identified the DHC gene cluster in Alternaria cinerariae, heterologously expressed the active HR PKS (Dhc3) and NR PKS (Dhc5) in yeast, and compared them to corresponding proteins that make DHC in Aspergillus terreus. Phylogenetic analysis and homology modeling of these enzymes identified variable surfaces and conserved motifs that are implicated in product formation.

Fungal biosynthesis of the bibenzoquinone oosporein to evade insect immunity.

Quinones are widely distributed in nature and exhibit diverse biological or pharmacological activities; however, their biosynthetic machineries are largely unknown. The bibenzoquinone oosporein was first identified from the ascomycete insect pathogen Beauveria bassiana>50 y ago. The toxin can also be produced by different plant pathogenic and endophytic fungi with an array of biological activities. Here, we report the oosporein biosynthetic machinery in fungi, a polyketide synthase (PKS) pathway including seven genes for quinone biosynthesis. The PKS oosporein synthase 1 (OpS1) produces orsellinic acid that is hydroxylated to benzenetriol by the hydroxylase OpS4. The intermediate is oxidized either nonenzymatically to 5,5'-dideoxy-oosporein or enzymatically to benzenetetrol by the putative dioxygenase OpS7. The latter is further dimerized to oosporein by the catalase OpS5. The transcription factor OpS3 regulates intrapathway gene expression. Insect bioassays revealed that oosporein is required for fungal virulence and acts by evading host immunity to facilitate fungal multiplication in insects. These results contribute to the known mechanisms of quinone biosynthesis and the understanding of small molecules deployed by fungi that interact with their hosts.

Polyketide synthases from poison hemlock (Conium maculatum L.).

Coniine is a toxic alkaloid, the biosynthesis of which is not well understood. A possible route, supported by evidence from labelling experiments, involves a polyketide formed by the condensation of one acetyl-CoA and three malonyl-CoAs catalysed by a polyketide synthase (PKS). We isolated PKS genes or their fragments from poison hemlock (Conium maculatum L.) by using random amplification of cDNA ends (RACE) and transcriptome analysis, and characterized three full-length enzymes by feeding different starter-CoAs in vitro. On the basis of our in vitro experiments, two of the three characterized PKS genes in poison hemlock encode chalcone synthases (CPKS1 and CPKS2), and one encodes a novel type of PKS (CPKS5). We show that CPKS5 kinetically favours butyryl-CoA as a starter-CoA in vitro. Our results suggest that CPKS5 is responsible for the initiation of coniine biosynthesis by catalysing the synthesis of the carbon backbone from one butyryl-CoA and two malonyl-CoAs.

The Biogeography of Putative Microbial Antibiotic Production.

Understanding patterns in the distribution and abundance of functional traits across a landscape is of fundamental importance to ecology. Mapping these distributions is particularly challenging for species-rich groups with sparse trait measurement coverage, such as flowering plants, insects, and microorganisms. Here, we use likelihood-based character reconstruction to infer and analyze the spatial distribution of unmeasured traits. We apply this framework to a microbial dataset comprised of 11,732 ketosynthase alpha gene sequences extracted from 144 soil samples from three continents to document the spatial distribution of putative microbial polyketide antibiotic production. Antibiotic production is a key competitive strategy for soil microbial survival and performance. Additionally, novel antibiotic discovery is highly relevant to human health, making natural antibiotic production by soil microorganisms a major target for bioprospecting. Our comparison of trait-based biogeographical patterns to patterns based on taxonomy and phylogeny is relevant to our basic understanding of microbial biogeography as well as the pressing need for new antibiotics.

Phylogeny of type I polyketide synthases (PKSs) in fungal entomopathogens and expression analysis of PKS genes in Beauveria bassiana BCC 2660.

Entomopathogenic fungi are able to invade and kill insects. Various secondary metabolites can mediate the interaction of a fungal pathogen with an insect host and also help the fungus compete with other microbes. Here we screened 23 isolates of entomopathogenic fungi for polyketide synthase (PKS) genes and amplified 72 PKS gene fragments using degenerate PCR. We performed a phylogenetic analysis of conserved ketosynthase and acyltransferase regions in these 72 sequences and 72 PKSs identified from four insect fungal genome sequences. The resulting genealogy indicated 47 orthologous groups with 99-100 % bootstrap support, suggesting shared biosynthesis of identical or closely related compounds from different fungi. Three insect-specific groups were identified among the PKSs in reducing clades IIa, IIb, and III, which comprised PKSs from 12, 9, and 30 fungal isolates, respectively. A IIa-IIb pair could be found in seven fungi. Expression analyses revealed that eleven out of twelve PKS genes identified in Beauveria bassiana BCC 2660 were expressed in culture. PKS genes from insect-specific clades IIa and IIb were expressed only in insect-containing medium, while others were expressed only in PDB or in CYB, PDB and SDY. The data suggest the potential production of several polyketides in culture.

Bioinformatical analysis of the sequences, structures and functions of fungal polyketide synthase product template domains.

The product template (PT) domains, specifically in fungal non-reducing polyketide synthases (NR-PKSs), mediate the regioselective cyclization of polyketides dominating the final structures. However, up to date, the systematic knowledge about PT domains has been insufficient. In present study, the relationships between sequences, structures and functions of the PT domains were analyzed with 661 NR-PKS sequences. Based on the phylogenetic analysis, the PT domains were classified into prominent eight groups (I-VIII) corresponding with the representative compounds and cyclization regioselectivity (C2-C7, C4-C9, and C6-C11). Most of the cavity lining residue (CLR) sites in all groups were common, while the regional CLR site mutations resulted in the appearance of finger-like regions with different orientation. The cavity volumes and shapes, even the catalytic dyad positions of PT domains in different groups were corresponding with characteristic cyclization regioselectivity and compound sizes. The conservative residues in PT sequences were responsible for the cyclization functions and the evolution of the key residues resulted in the differentiations of cyclization functions. The above findings may help to better understand the cyclization mechanisms of PT domains and even predict the structural types of the aromatic polyketide products.

An update to polyketide synthase and non-ribosomal synthetase genes and nomenclature in Fusarium.

Members of the genus Fusarium produce a plethora of bioactive secondary metabolites, which can be harmful to humans and animals or have potential in drug development. In this study we have performed comparative analyses of polyketide synthases (PKSs) and non-ribosomal peptide synthetases (NRPSs) from ten different Fusarium species including F. graminearum (two strains), F. verticillioides, F. solani, F. culmorum, F. pseudograminearum, F. fujikuroi, F. acuminatum, F. avenaceum, F. equiseti, and F. oxysporum (12 strains). This led to identification of 52 NRPS and 52 PKSs orthology groups, respectively, and although not all PKSs and NRPSs are assumed to be intact or functional, the analyses illustrate the huge secondary metabolite potential in Fusarium. In our analyses we identified a core collection of eight NRPSs (NRPS2-4, 6, 10-13) and two PKSs (PKS3 and PKS7) that are conserved in all strains analyzed in this study. The identified PKSs and NRPSs were named based on a previously developed classification system (www.FusariumNRPSPKS.dk). We suggest this system be used when PKSs and NRPSs have to be classified in future sequenced Fusarium strains. This system will facilitate identification of orthologous and non-orthologous NRPSs and PKSs from newly sequenced Fusarium genomes and will aid the scientific community by providing a common nomenclature for these two groups of genes/enzymes.