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Apolipoprotein A-Ⅰ (ApoA-Ⅰ) is the principal protein component of plasma high-density lipoprotein (HDL).It plays an important role in lipid transport and uptake and has been demonstrated to defend against bacteria in vertebrates.In this report, we identified a cDNA sequence from E.akaara ApoA-Ⅰ with an ORF of 792 bp encoding a putative polypeptide of 263 amino acid residues that shares some common features with known vertebrate ApoA-Ⅰ sequences.Tissue distribution analysis indicated that ApoA-I mRNA is broadly expressed in all tissues tested, including the fat metabolic organs (liver and intestine).We successfully expressed the ApoA-Ⅰ gene using a pET28a(+) vector and acquired a highly purified protein that showed potent antimicrobial activity against the Gram-positive bacterium S.aureus as well as the Gram-negative bacteria E.coli, P.damselae and V.harveyi, indicating that ApoA-Ⅰ may have a broad-spectrum antibacterial property to fish innate immunity.To determine the ApoA-Ⅰ response to lipid metabolism, we analyzed the ApoA-Ⅰ mRNA levels in the liver and intestine after feeding.The results indicate that intestinal ApoA-Ⅰ becomes involved in fat metabolism approximately one hour (h) after feeding, followed by hepatic ApoA-Ⅰ approximately 4 to 6 h post-feeding.Diversified types and proportions of dietary fatty acids did not affect the short-term levels of ApoA-Ⅰ mRNA or plasma HDL-cholesterol in E.akaara.A chronic high-fat diet did not impact hepatic ApoA-Ⅰ mRNA, although it decreased both intestinal ApoA-Ⅰ mRNA and the plasma HDL-cholesterol levels.In addition, chronic high-fat diets caused fat deposition in hepatic tissues, which is most likely harmful to fish health.This work provides a unique basis for substantial future work and will be helpful to develop a successful diet for fish culture.