Aspergillus oryzae 3.042 was mutagenized using atmospheric and room-temperature plasma (ARTP) technology to enhance its salt-tolerant proteases activity. Compared to the starting strain, mutant H8 subjected to 180 s of ARTP treatment exhibited excellent genetic stability (15 generations), growth rate, and significantly increased activities of neutral proteases, alkaline proteases, and aspartyl aminopeptidase during fermentation. Mutant H8 significantly enhanced the contents of 1–5 kDa peptides, aspartic acid, serine, threonine, and cysteine in soy sauce by 16.61, 7.69, 17.30, 8.61, and 45.00%, respectively, but it had no effects on the contents of the other 14 free amino acids (FAAs) due to its slightly enhanced acidic proteases activity. Analyses of transcriptional expressions of salt-tolerant alkaline protease gene (AP, gi: 217809) and aspartyl aminopeptidase gene (AAP, gi: 6165646) indicated that their expression levels were increased by approximately 30 and 27%, respectively. But no mutation was found in the sequences of AP and AAP expression cassettes, suggesting that the increased activities of proteases in mutant H8 should be partially attributed to the increased expression of proteases. ARTP technology showed great potential in enhancing the activities of salt-tolerant proteases from A. oryzae.

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BACKGROUND The salt tolerance of proteases secreted by Aspergillus oryzae 3.042 closely relates to the utilization of raw materials and the quality of soy sauce. However, little is known about the salt‐tolerant proteases and their salt‐tolerant mechanisms. RESULTS In this study, we isolated and identified a salt‐tolerant alkaline protease (AP, approximately 29 kDa) produced by A. oryzae 3.042. It was considered as a metal‐ion‐independent serine protease. The optimum and stable pH values were both pH 9.0 and the optimum temperature was 40 °C. Over 20% relative activity of AP remained in the presence of 3.0 mol L−1 NaCl after 7 days, but its Km and Vmax were only mildly influenced by the presence of 3.0 mol L−1 NaCl, indicating its outstanding salt tolerance. Furthermore, AP was more stable than non‐salt‐tolerant protease at high salinity. The salt‐tolerant mechanisms of AP could be due to more salt bridges, higher proportion of ordered secondary structures and stronger hydrophobic amino acid residues in the interior. CONCLUSIONS The above results are vital for maintaining, activating and/or modulating the activity of AP in high‐salt environments. They would also provide theoretical guidance for the modification of AP and the engineering of A. oryzae 3.042 so as to secrete more AP. © 2018 Society of Chemical Industry

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