Abstract:
An actinomycete, designated strain AA8, isolated from soil collected from the orchard in Bangkok province, was identified using a polyphasic taxonomic approach. The result showed that strain AA8 was a member of the genus Streptomyces and was closely
related to Streptomyces roseifaciens DSM 1 0 6 1 9 6 T with the highest 16S rRNA gene sequence similarity (99.9 %). Genomic-based identification revealed strain AA8 was most closely related to Streptomyces roseifaciens DSM 106196T and shared the low values of
average nucleotide identity (ANI) (94.1%). Genome analysis by antiSMASH platform revealed that strain AA8 contained several secondary metabolite biosynthetic gene clusters (smBGCs) i.e. type I and III polyketide synthase gene clusters, terpene biosynthesis
gene clusters, non-ribosomal peptide synthetase (NRPS) gene clusters, and NRPS-like gene clusters. The crude extract of strain AA8 exhibited antioxidant activity at IC₅₀ 7.03 ± 0.02 mg/ml. Therefore, the chemical investigation was conducted by employing bioassayguided method and led to the isolation of 9 pure compounds, including 3,4-dimethoxypropiophenone (A), 6,7,8-trimethoxy-3-methylisocoumarin (B), 3, 4-dimethoxybenzamide (C), 6,8-dimethoxy-3-methylisocoumarin (D), 4-hydroxy-3-methoxybenzamide (E), 3,4-dihydroxybenzaldehyde (F), Nb- acetyltryptamine (G), 3-Hydroxymethyl-6,7,8-trimethoxyisocoumarin (H), 1-methoxy-1H-pyrrole-2-carboxamide (I). These isolated compounds were evaluated for antioxidant activity. All compounds exhibited antioxidant activity in the range of IC₅₀ 0.87384.85 mg/ml. Besides, compounds 3,4-dihydroxybenzaldehyde and 6,7,8- trimethoxy-3-methylisocoumarin displayed strong antioxidant activity, indicating a potential for antioxidant agents.