Marine actinobacteria are drawing more and more attention as a promising source of new natural products. Here we report isolation, genome sequencing and metabolic profiling of new strain Streptomyces sp. MP131-18 isolated from marine sediment sample collected in the Trondheim Fjord, Norway. The 16S rRNA and multilocus phylogenetic analysis showed that MP131-18 belongs to the genus Streptomyces. The genome of MP131-18 isolate was sequenced, and 36 gene clusters involved in the biosynthesis of 18 different types of secondary metabolites were predicted using antiSMASH analysis. The combined genomics-metabolics profiling of the strain led to the identification of several new biologically active compounds. As a result, the family of bisindole pyrroles spiroindimicins was extended with two new members, spiroindimicins E and F. Furthermore, prediction of the biosynthetic pathway for unusual α-pyrone lagunapyrone isolated from MP131-18 resulted in foresight and identification of two new compounds of this family – lagunapyrones D and E. The diversity of identified and predicted compounds from Streptomyces sp. MP131-18 demonstrates that marine-derived actinomycetes are not only a promising source of new natural products, but also represent a valuable pool of genes for combinatorial biosynthesis of secondary metabolites.

The environment of Lake Baikal is a well-known source of microbial diversity. The strain Streptomyces sp. IB2014/011-12, isolated from samples collected at Lake Baikal, was found to exhibit potent activity against Gram-positive bacteria. Here, we report isolation and characterization of linear polyketide alpiniamide A (1) and its new derivatives B–D (2–5). The structures of alpiniamides A–D were established and their relative configuration was determined by combination of partial Murata’s method and ROESY experiment. The absolute configuration of alpiniamide A was established through Mosher’s method. The gene cluster, responsible for the biosynthesis of alpiniamides (alp) has been identified by genome mining and gene deletion experiments. The successful expression of the cloned alp gene cluster in a heterologous host supports these findings. Analysis of the architecture of the alp gene cluster and the feeding of labeled precursors elucidated the alpiniamide biosynthetic pathway. The biosynthesis of alpiniamides is an example of a rather simple polyketide assembly line generating unusual chemical diversity through the combination of domain/module skipping and double bond migration events.

Genome mining of actinomycete bacteria aims at the discovery of novel bioactive secondary metabolites that can be developed into drugs. A new repressorbased biosensor to detect activated secondary metabolite biosynthesis gene clusters in Streptomyces was developed. Biosynthetic gene clusters for undecylprodigiosin and coelimycin in the genome of Streptomyces lividans TK24, which encoded TetR-like repressors and appeared to be almost "silent" based on the RNA-seq data, were chosen for the proof-of-principle studies. The bpsA reporter gene for indigoidine synthetase was placed under control of the promotor/operator regions presumed to be controlled by the cluster-associated TetR-like repressors. While the biosensor for undecylprodigiosin turned out to be nonfunctional, the coelimycin biosensor was shown to perform as expected, turning on biosynthesis of indigoidine in response to the concomitant production of coelimycin. The developed reporter system concept can be applied to those cryptic gene clusters that encode metabolite-sensing repressors to speed up discovery of novel bioactive compounds in Streptomyces.

Metabolic profiling of Streptomyces sp.IB2014/016-6l ed to the identification of three new tetrahydroisoquinoline natural products,p erquinolines A-C (1-3). Labelled precursor feeding studies and the cloning of the pqr biosynthetic gene cluster revealed that 1-3 are assembled by the action of several unusual enzymes.The biosynthesis starts with the condensation of succinyl-CoA and l-phenylalanine catalyzedbythe amino-7-oxononanoate synthase-like enzyme PqrA, representing rare chemistry in natural product assembly.T he second condensation and cyclization events are conducted by PqrG,anenzyme resembling an acyl-CoA ligase.L ast, ATP-grasp RimK-type ligase PqrI completes the biosynthesis by transferring a gaminobutyric acid or b-alanine moiety.T he discovered pathway represents an ew route for assembling the tetrahydroisoquinoline cores of natural products.

Natural products produced by bacteria found in unusual and poorly studied ecosystems, such as Lake Baikal, represent a promising source of new valuable drug leads. Here we report the isolation of a new Streptomyces sp. strain IB201691-2A from the Lake Baikal endemic mollusk Benedictia baicalensis. In the course of an activity guided screening three new angucyclines, named baikalomycins A–C, were isolated and characterized, highlighting the potential of poorly investigated ecological niches. Besides that, the strain was found to accumulate large quantities of rabelomycin and 5-hydroxy-rabelomycin, known shunt products in angucyclines biosynthesis. Baikalomycins A–C demonstrated varying degrees of anticancer activity. Rabelomycin and 5-hydroxy-rabelomycin further demonstrated antiproliferative activities. The structure elucidation showed that baikalomycin A is a modified aquayamycin with β-d-amicetose and two additional hydroxyl groups at unusual positions (6a and 12a) of aglycone. Baikalomycins B and C have alternating second sugars attached, α-l-amicetose and α-l-aculose, respectively. The gene cluster for baikalomycins biosynthesis was identified by genome mining, cloned using a transformation-associated recombination technique and successfully expressed in S. albus J1074. It contains a typical set of genes responsible for an angucycline core assembly, all necessary genes for the deoxy sugars biosynthesis, and three genes coding for the glycosyltransferase enzymes. Heterologous expression and deletion experiments allowed to assign the function of glycosyltransferases involved in the decoration of baikalomycins aglycone.

Natural products are an important source of novel investigational compounds in drug discovery. Especially in the field of antibiotics, Actinobacteria have been proven to be a reliable source for lead structures. The discovery of these natural products with activity- and structure-guided screenings has been impeded by the constant rediscovery of previously identified compounds. Additionally, a large discrepancy between produced natural products and biosynthetic potential in Actinobacteria, including representatives of the order Pseudonocardiales, has been revealed using genome sequencing. To turn this genomic potential into novel natural products, we used an approach including the in-silico pre-selection of unique biosynthetic gene clusters followed by their systematic heterologous expression. As a proof of concept, fifteen Saccharothrixespanaensis genomic library clones covering predicted biosynthetic gene clusters were chosen for expression in two heterologous hosts, Streptomyceslividans and Streptomycesalbus. As a result, two novel natural products, an unusual angucyclinone pentangumycin and a new type II polyketide synthase shunt product SEK90, were identified. After purification and structure elucidation, the biosynthetic pathways leading to the formation of pentangumycin and SEK90 were deduced using mutational analysis of the biosynthetic gene cluster and feeding experiments with 13C-labelled precursors.

Pyrethrum pulchrum Ledeb. has been a phytochemically unexplored Mongolian medicinal folklore plant. In this study, its total flavonoid content was determined and fourteen flavonoids (1-14) were isolated from the aerial parts of P. pulchrum. Their structures were elucidated on the basis of spectroscopic data. The compounds 12-14, methoxyflavones, were tested for antiproliferative and cytotoxic activity against A549, HeLa, K-562, THP-1 and HUVEC cell lines. This is the first report on the effects of 5,7,4'-trihydroxy-3,6,3'-trimethoxyflavone ( 13) against all tested cell lines and it exhibited potent activity against chronic myeloid leukemia K-562 and acute monocytic leukemia THP-1 cells, each with GI 50 value at 2.0 μg/mL. The 5,4 'dihydroxy-3,6,7,3'-tetramethoxyflavone (14) showed the most potent activity against THP-1 (GI 50 = 1.1 μg/mL) and the highest cytotoxicity (5.6 μg/mL). In addition, acute toxicity of plant ethanol extract was evaluated and the lethal dose (LD 50 ) was estimated at 1058 mg/kg.

Natural products derived from plants, fungi or bacteria have been used for years in the medicine, agriculture and food industries as they exhibit a variety of beneficial properties, such as antibiotic, antifungal, anticancer, herbicidal and immunosuppressive activities. Compared to synthetic compounds, natural products possess a greater chemical diversity, which is a reason why they are profitable templates for developing pharmaceutical drug candidates and ongoing research on them is inevitable. Performing heterologous expression with unknown gene clusters is the preferred method to activate gene clusters that are not expressed in the wild-type strain under laboratory conditions; thus, this method offers a way to discover new interesting metabolites. Here, we report the gene cluster assembly of a hybrid NRPS-PKS gene cluster from Streptomyces mirabilis Lu17588, which was heterologously expressed in Streptomyces albus Del14. Four new compounds were produced by the obtained strain, which were named miramides A–D. Isolation and structure elucidation revealed similarity of the isolated compounds to the known depsipeptides rimosamides/detoxins.