Draft Genome Sequence of the Xanthocidin-Producing Strain Streptomyces sp. AcE210, Isolated from a Root Nodule of Alnus glutinosa (L.).

Streptomyces sp. strain AcE210 exhibited antibacterial activity toward Gram-positive microorganisms and turned out to be a rare producer of the specialized metabolite xanthocidin. The 10.6-Mb draft genome sequence gives insight into the complete specialized metabolite production capacity and builds the basis to find and locate the biosynthetic gene cluster of xanthocidin.

Xanthocidin Derivatives from the Endophytic Streptomyces sp. AcE210 Provide Insight into Xanthocidin Biosynthesis.

Xanthocidin and six new derivatives were isolated from the endophytic Streptomyces sp. AcE210. Their planar structures were elucidated by 1D and 2D NMR spectroscopy as well as by HRMS. The absolute configuration of one compound was determined by using vibrational circular dichroism spectroscopy (VCD). The structural similarities of xanthocidin and some of the isolated xanthocidin congeners to the methylenomycins A, B, and C suggested that the biosynthesis of these compounds might follow a similar route. Feeding studies with isotopically labelled [13 C5 ]-l-valine showed that instead of utilizing acetyl-CoA as starter unit, which has been proposed for the methylenomycin biosynthesis, Streptomyces sp. AcE210 employs an isobutyryl-CoA starter unit, resulting in a branched side chain in xanthocidin. Further evidence for a comparable biosynthesis was given by the analysis of the genome sequence of Streptomyces sp. AcE210 that revealed a cluster of homologues to the mmy genes involved in methylenomycin biosynthesis.

Streptomyces AcH 505 triggers production of a salicylic acid analogue in the fungal pathogen Heterobasidion abietinum that enhances infection of Norway spruce seedlings.

The necrotrophic fungus Heterobasidion spp. is the causal agent of 'annosum root rot' of Norway spruce. In the presence of the rhizosphere bacterium Streptomyces AcH 505, enhanced colonization of Norway spruce roots with Heterobasidion abietinum 331 has previously been observed. By analyzing dual cultures of H. abietinum 331 and Streptomyces AcH 505 with HPLC, a fungal metabolite was identified that was increased in the presence of Streptomyces AcH 505. Likewise, challenge of H. abietum 331 with common antifungals produced by soil streptomycetes rendered the same effect. The structure of the compound, 5-formylsalicylic acid (5-FSA), was elucidated by HPLC-HR-ESI-Orbitrap-mass spectrometry and NMR spectroscopy. Based on in vivo measurements of maximum photosystem II efficiency of Norway spruce seedlings, 5-FSA did not influence plant vitality. However, when challenged with H. abietinum 331, ergosterol amounts in infected roots increased significantly for 5-FSA pre-treated seedlings. The severity of the infection was comparable to that observed in the presence of Streptomyces AcH 505. 5-FSA is a structural analogue of salicylic acid, an important signalling molecule active in plant defence. Thus, the expression of two defence-response related marker genes (PR1, Hel) was analysed in 5-FSA treated Arabidopsis thaliana seedlings by Northern blot analysis. The transcription of both marker genes was altered, indicating that 5-FSA is perceived by Arabidopsis in a similar manner to salicylic acid and is able to interfere with Arabidopsis defence signalling. The role of 5-FSA as a potential virulence factor of H. abietinum 331 in the presence of Streptomyces AcH 505 is discussed.