The cyclic depsipeptide skyllamycin A is a potent inhibitor of the platelet-derived growth factor (PDGF) signaling pathway by inhibiting binding of homodimeric PDGF BB to the PDGF β-receptor. Its structure contains a cinnamoyl side chain and shows a high amount of β-hydroxylated amino acids as well as an unusual α-hydroxyglycine moiety as a rare structural modification. The skyllamycin biosynthetic gene cluster was cloned and sequenced from Streptomyces sp. Acta 2897. Its analysis revealed the presence of open reading frames encoding proteins for fatty acid precursor biosynthesis, non-ribosomal peptide synthetases, regulators, and transporters along with other modifying enzymes. Specific in-frame mutagenesis of these tailoring enzymes resulted in the production of novel skyllamycin derivatives revealing that β-hydroxy groups in skyllamycin A are introduced by a promiscuous cytochrome P450 monooxygenase, whereas a two-component flavin-dependent monooxygenase is involved in α-hydroxylation.

New structurally diverse natural products are discovered when novel screening procedures are introduced or when high quality biological materials from new sources are examined in existing screens, hence it is important to foster these two aspects of novelty in drug discovery programmes. Amongst prokaryotes, actinomycetes, notably streptomycetes, remain a rich source of new natural products though it has become increasingly difficult to find such metabolites from common actinomycetes as screening 'old friends' leads to the costly rediscovery of known compounds. The bioprospecting strategy which is the subject of this review is based upon the premise that new secondary metabolites can be found by screening relatively small numbers of dereplicated, novel actinomycetes isolated from marine sediments. The success of the strategy is exemplified by the discovery of a range of novel bioactive compounds, notably atrop-abyssomicin C and proximicins A, B and C from Verrucosispora strains isolated from sediment samples taken from the Sea of Japan and the Raune Fjord, respectively, and the dermacozines derived from Dermacoccus strains isolated from the Challenger Deep of the Mariana Trench in the Pacific Ocean. The importance of current advances in prokaryotic systematics in work of this nature is stressed and a plea made that resources be sought to train, support and employ the next generation of actinobacterial systematists.

A screening method was established to detect inhibitors of the biosynthetic pathways of aromatic amino acids and para-aminobenzoic acid, the precursor of folic acid, using an agar A successful search for novel antibacterial metabolites has to meet three criteria, first, a specific target which is essential for the metabolism of a bacterium and not yet provided with an known inhibitor. Second, a set of taxonomically characterized and dereplicated microorganisms as producers of secondary metabolites, and last but not least a lucky but experienced hand for strain isolation and cultivation. We have chosen the shikimate pathway as an essential target of bacterial metabolism, with special consideration of the biosynthesis of aromatic amino acids and para-aminobenzoic (pAba) acid derived from the keymetabolite chorismate. Only a few antimetabolites are known as inhibitors of aromatic amino acids, such as L-2,5-dihydrophenylalanine2), an antagonist of phenlyalanine, and glyphosate that inhibits 3-enolpyruvylshikimate-3-phosphate synthase3,4). To our knowledge, no natural product inhibitor of pAba biosynthesis has been described in the literature. This pathway, which is catalyzed by two enzymes, 4-amino-4-deoxychorismic acid (ADC) synthase and ADC lyase, seems to be of considerable interest for the development of novel antibiotics since it is directly linked to folic acid biosynthesis, which is established in plants, fungi, prokaryotes and parasites of the apicomplexa group (Plasmodium, Toxoplasma) but not in vertebrates.As suitable producers of bioactive metabolites we screened within the order Actinomycetales terrestrial and marine members of the families Streptomycetaceae and Micromonosporaceae and rare actinomycete genera. A total of 930 extracts derived from 201 actinomycetes were subjected to the screening. Among them, only AB-18-032, an extract from a marine isolate from a sediment collected from the Sea of Japan, was found to exhibit activity against

Plumbing the depths: Abyssomicin C (structure shown) from ocean floor sediment is a novel antibiotic that inhibits the biosynthesis steps between chorismate and p‐aminobenzoic acid. Its activity may be explained by the irreversible trapping of the targeted enzymes by a Michael addition. Blocking the biosynthesis of p‐aminobenzoic acid may be one approach to developing new antibiotics.

The mycorrhiza helper bacterium Streptomyces strain AcH 505 improves mycelial growth of ectomycorrhizal fungi and formation of ectomycorrhizas between Amanita muscaria and spruce but suppresses the growth of plant-pathogenic fungi, suggesting that it produces both fungal growth-stimulating and -suppressing compounds. The dominant fungal-growth-promoting substance produced by strain AcH 505, auxofuran, was isolated, and its effect on the levels of gene expression of A. muscaria was investigated. Auxofuran and its synthetic analogue 7-dehydroxy-auxofuran were most effective at a concentration of 15 M, and application of these compounds led to increased lipid metabolism-related gene expression. Cocultivation of strain AcH 505 and A. muscaria stimulated auxofuran production by the streptomycete. The antifungal substances produced by strain AcH 505 were identified as the antibiotics WS-5995 B and C. WS-5995 B completely blocked mycelial growth at a concentration of 60 M and caused a cell stress-related gene expression response in A. muscaria. Characterization of these compounds provides the foundation for molecular analysis of the fungus-bacterium interaction in the ectomycorrhizal symbiosis between fly agaric and spruce.Actinomycetes are known for their capacity to control plant diseases. A number of investigations have reported antagonism of root-pathogenic fungi by soil actinomycetes (6, 40) and that especially streptomycetes are a rich source of antifungal compounds (20).Ectomycorrhizas (ECM) are widespread symbiotic organs that are formed during interactions between most tree species or perennials and soil fungi (44). The establishment of the symbiosis is affected by other soil microbes, such as bacteria and other fungi (18,29). Understanding the interactions between the soil-inhabiting microorganisms in the mycorrhizosphere is very important, as they are key players in nutrient cycling in forest soils (29).Under natural conditions, the plant partner is able to select for bacterial strains that are beneficial for the ECM symbiosis and for plant growth (16). Specifically, the growth of ectomycorrhizal fungi and mycorrhiza formation are promoted by some of the mycorrhizosphere bacteria (mycorrhiza helper bacteria [MHB] [18]), which belong to the genera Bacillus, Burkholderia, Pseudomonas, Rhodococcus, and Streptomyces (15,19,30,37,42).When 12 actinomycete isolates were tested to determine their effects on mycelial growth of ectomycorrhizal fungi (39), the bacterial isolates inhibited, promoted, or had no significant effect on hyphal extension in dual cultures. Thus, there is an opportunity to select for actinomycetes that stimulate certain symbiotic fungi and plant growth but are antagonistic to pathogenic organisms. To do this, we isolated a collection of actinomycetes from the hyphosphere of a spruce (Picea abies) stand and examined their effects on symbiotic and plant-pathogenic fungi (30,31). One member of this collection, streptomycete strain AcH 505, exhibited the desired qualities, as it significantly pro...