An Antifungal Protein from the Marine Bacterium Streptomyces sp. Strain AP77 Is Specific for Pythium porphyrae , a Causative Agent of Red Rot Disease in Porphyra spp

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Abstract
A novel antifungal protein (SAP) was found in the culture supernatant of a marine bacterium, Streptomyces sp. strain AP77, and was purified. This protein was characterized by chemical, biochemical, and biological analyses. By using gel filtration, the molecular mass of SAP was estimated to be 160 kDa. Structural analysis of SAP by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and matrix-assisted laser desorption ionization-time of flight mass spectrometry suggested that SAP is composed of three heterologous protein subunits of 41.7 kDa (SAP1), 21.7 kDa (SAP2), and 18.7 kDa (SAP3) at a molar ratio of 1:1:5 (or 1:1:6). N-terminal amino acid sequence analysis and a homology search revealed that SAP1, SAP2, and SAP3 exhibit 64.3, 68.4, and 86.7% similarity to three Streptomyces coelicolor polypeptides, puromycin resistance protein (Pur8), a conserved hypothetical protein, and bacterioferritin, respectively. The MIC of purified SAP against Pythium porphyrae was determined to be 1.6 μg/disk, whereas no inhibitory effect was observed at concentrations up to 100 μg/disk against most of the fungal and bacterial strains tested; the only exception was relatively strong antifungal activity against Pythium ultimum (MIC, 6.3 μg/disk). In vitro and in vivo toxicity tests demonstrated that SAP showed no toxicity against Porphyra yezoensis cells, human normal dermal fibroblasts, and mice at doses up to 700 μg/ml (for 24 h), 250 μg/ml (for 12 h), and 75 mg/kg (for 35 days), respectively. SAP was labile when it was subjected to a heated-air drying treatment, which is a great advantage in food production procedures. These results indicated that Streptomyces sp. strain AP77 might be useful as a gene source for safe transgenic Porphyra breeding for tolerance to Pythium infection.

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