Streptomyces lydicamycinicus sp. nov. and Its Secondary Metabolite Biosynthetic Gene Clusters for Polyketide and Nonribosomal Peptide Compounds

Heterologous expression and enzymological characterization of L-glutamate oxidase from the marine actinomycete Streptomyces lydicamycinicus NBRC 110027

We successfully constructed a heterologous expression system for L-glutamate oxidase from the marine actinomycete Streptomyces lydicamycinicus NBRC 110027 (Sl-LGOX) in Escherichia coli BL21(DE3) as a host. This is the first example of L-glutamate oxidase from a marine microorganism. A chemically synthesized gene optimized for codon usage in E. coli was used as the inserted fragment, which was effective for enzyme expression. We expressed Sl-LGOX in the soluble fraction of E. coli BL21(DE3)/pET21b-Sl-lgox. We also succeeded in purifying the recombinant Sl-LGOX (rSl-LGOX) to homogeneity from the cell-free extract of this clone via an Ni-NTA column. rSl-LGOX showed high specificity for L-Glu and was active and stable over a wide range of temperatures and pH values. In particular, it showed high specific activity and stability at an acidic pH. A variety of applications can take advantage of the unique enzymatic properties of rSl-LGOX.

Genomic and Phenotypic Characterization of Streptomyces sirii sp. nov., Amicetin-Producing Actinobacteria Isolated from Bamboo Rhizospheric Soil

In our large-scale search for antimicrobial-producing bacteria, we isolated an actinomycete strain from rhizospheric soil of Bambusa vulgaris. The strain designated BP-8 showed noticeable antibacterial activity. BP-8 was subjected to a whole-genome analysis via a polyphasic taxonomy approach, and its antibacterial metabolite was identified by HRLS-MS. The results of the physiological and morphological analyses indicated that BP-8 is an aerobic, neutrophilic, mesophilic organism that is tolerant to 8% NaCl and can use a wide range of carbohydrates. It forms curly sporophores with a warty surface. The results of the phylogenetic and average nucleotide identity analyses and in silico DNA-DNA hybridization calculation indicated that BP-8 represents the type strain of a novel Streptomyces species. A comparative in silico analysis of the genome sequences of BP-8 and its closest related strains revealed the presence of genes encoding chemotaxonomic markers characteristic of Streptomyces. The antibacterial compound was identified as amicetin. Genomic mining also revealed more than 10 biosynthetic gene clusters that have not been described previously and may lead to the discovery of new valuable compounds. On the basis of these results, strain BP-8T (=VKM Ac-3066T = CCTCC AA 2024094T) is proposed as the type strain of the novel species Streptomyces sirii sp. nov.

Development of a drug discovery approach from microbes with a special focus on isolation sources and taxonomy

After the successful discoveries of numerous antibiotics from microorganisms, frequent reisolation of known compounds becomes an obstacle in further development of new drugs from natural products. Exploration of biological sources that can provide novel scaffolds is thus an urgent matter in drug lead screening. As an alternative source to the conventionally used soil microorganisms, we selected endophytic actinomycetes, marine actinomycetes, and actinomycetes in tropical areas for investigation and found an array of new bioactive compounds. Furthermore, based on the analysis of the distribution pattern of biosynthetic gene clusters in bacteria together with available genomic data, we speculated that biosynthetic gene clusters for secondary metabolites are specific to each genus. Based on this assumption, we investigated actinomycetal and marine bacterial genera from which no compounds have been reported, which led to the discovery of a variety of skeletally novel bioactive compounds. These findings suggest that consideration of environmental factor and taxonomic position is critically effective in the selection of potential strains producing structurally unique compounds.

Recent Progress of Reclassification of the Genus Streptomyces

The genus Streptomyces is a representative group of actinomycetes and one of the largest taxa in bacteria, including approximately 700 species with validly published names. Since the classification was mainly based on phenotypic characteristics in old days, many members needed to be reclassified according to recent molecular-based taxonomies. Recent developments of molecular-based analysis methods and availability of whole genome sequences of type strains enables researchers to reclassify these phylogenetically complex members on a large scale. This review introduces reclassifications of the genus Streptomyces reported in the past decade. Appropriately 34 Streptomyces species were transferred to the other genera, such as Kitasatospora, Streptacidiphilus, Actinoalloteichus and recently proposed new genera. As a result of reclassifications of 14 subspecies, the genus Streptomyces includes only four subspecies at present in practice. A total of 63 species were reclassified as later heterotypic synonyms of previously recognized species in 24 published reports. As strong relationships between species and the secondary metabolite-biosynthetic gene clusters become clarified, appropriate classifications of this genus will not only contribute to systematics, but also provide significant information when searching for useful bioactive substances.

Differences at Species Level and in Repertoires of Secondary Metabolite Biosynthetic Gene Clusters among Streptomyces coelicolor A3(2) and Type Strains of S. coelicolor and Its Taxonomic Neighbors

Streptomyces coelicolor A3(2) is used worldwide for genetic studies, and its complete genome sequence was published in 2002. However, as the whole genome of the type strain of S. coelicolor has not been analyzed, the relationship between S. coelicolor A3(2) and the type strain is not yet well known. To clarify differences in their biosynthetic potential, as well as their taxonomic positions, we sequenced whole genomes of S. coelicolor NBRC 12854T and type strains of its closely related species—such as Streptomyces daghestanicus, Streptomyces hydrogenans, and Streptomyces violascens—via PacBio. Biosynthetic gene clusters for polyketides and non-ribosomal peptides were surveyed by antiSMASH, followed by bioinformatic analyses. Type strains of Streptomyces albidoflavus, S. coelicolor, S. daghestanicus, S. hydrogenans, and S. violascens shared the same 16S rDNA sequence, but S. coelicolor A3(2) did not. S. coelicolor A3(2) and S. coelicolor NBRC 12854T can be classified as Streptomycesanthocyanicus and S. albidoflavus, respectively. In contrast, S. daghestanicus, S. hydrogenans, and S. violascens are independent species, despite their identical 16S rDNA sequences. S. coelicolor A3(2), S. coelicolor NBRC 12854T, S. daghestanicus NBRC 12762T, S. hydrogenans NBRC 13475T, and S. violascens NBRC 12920T each harbor specific polyketide synthase (PKS) and non-ribosomal peptide synthetase (NRPS) gene clusters in their genomes, whereas PKS and NRPS gene clusters are well conserved between S. coelicolor A3(2) and S. anthocyanicus JCM 5058T, and between S. coelicolor NBRC 12854T and S. albidoflavus DSM 40455T, belonging to the same species. These results support our hypothesis that the repertoires of PKS and NRPS gene clusters are different between different species.

Reclassification of 15 Streptomyces species as synonyms of Streptomyces albogriseolus, Streptomyces althioticus, Streptomyces anthocyanicus, Streptomyces calvus, Streptomyces griseoincarnatus, Streptomyces mutabilis, Streptomyces pilosus or Streptomyces rochei

Taxonomic relationships in eight sets of Streptomyces species, (1a) Streptomyces enissocaesilis, Streptomyces plicatus, Streptomyces rochei and Streptomyces vinaceusdrappus, (1b) Streptomyces geysiriensis, (2) Streptomyces luteus and Streptomyces mutabilis, (3) Streptomyces flavoviridis and Streptomyces pilosus, (4) Streptomyces asterosporus and Streptomyces calvus, (5) Streptomyces erythrogriseus, Streptomyces griseoincarnatus, Streptomyces labedae and Streptomyces variabilis, (6a) Streptomyces griseorubens, (6b) Streptomyces matensis, (6c) Streptomyces althioticus, (7) Streptomyces albogriseolus and Streptomyces viridodiastaticus, (8a) Streptomyces humiferus and Streptomyces violaceolatus, (8b) Streptomyces anthocyanicus, Streptomyces coelescens and Streptomyces violaceoruber, were investigated. Type strains within each subset of 1a to 8b shared completely identical 16S rRNA gene sequences. In MLSA, subsets 1a and 1b, 6a to 6c, and 8a and 8b formed an independent clade, respectively, but the evolutionary distances between S. violaceoruber and the other members in set 8 and between S. griseorubens and those in set 6 were 0.022-0.023 and 0.0064-0.0076, respectively. Members in each of the other sets, except for S. labedae, formed an independent clade. In each clade, evolutionary distances between/among the members were <0.007 except for that between S. griseorubens and S. matensis in set 6, suggesting the same species. Digital DNA-DNA relatedness using whole genome sequences and phenotypic similarities supported the synonymies of sets 1 to 3, set 4 except for S. labedae, sets 5 to 7, and set 8 except for S. violaceoruber, respectively. Therefore, S. enissocaesilis, S. geysiriensis, S. plicatus and S. vinaceusdrappus were considered as later heterotypic synonyms of S. rochei; S. luteus as that of S. mutabilis; S. flavoviridis as that of S. pilosus; S. asterosporus as that of S. calvus; S. erythrogriseus and S. variabilis as those of S. griseoincarnatus; S. griseorubens and S. matensis as that of S. althioticus; S. viridodiastaticus as that of S. albogriseolus; S. coelescens, S. humiferus and S. violaceolatus as those of S. anthocyanicus.

Polyketide Synthase and Nonribosomal Peptide Synthetase Gene Clusters in Type Strains of the Genus Phytohabitans

(1) Background: Phytohabitans is a recently established genus belonging to rare actinomycetes. It has been unclear if its members have the capacity to synthesize diverse secondary metabolites. Polyketide and nonribosomal peptide compounds are major secondary metabolites in actinomycetes and expected as a potential source for novel pharmaceuticals. (2) Methods: Whole genomes of Phytohabitans flavus NBRC 107702^T, Phytohabitans rumicis NBRC 108638^T, Phytohabitans houttuyneae NBRC 108639^T, and Phytohabitans suffuscus NBRC 105367^T were sequenced by PacBio. Polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) gene clusters were bioinformatically analyzed in the genome sequences. (3) Results: These four strains harbored 10, 14, 18 and 14 PKS and NRPS gene clusters, respectively. Most of the gene clusters were annotated to synthesis unknown chemistries. (4) Conclusions: Members of the genus Phytohabitans are a possible source for novel and diverse polyketides and nonribosomal peptides.