Molecular cloning and functional characterization of a novel i-type lysozyme in the freshwater musselCristaria plicata

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The freshwater bivalveCristaria plicata, which is widely distributed in Eastern Asia, is a key species in the pearl culture industry. In this study, a novel invertebrate-type lysozyme, designated as CpLYZ2, was cloned from hemocytes ofC. plicata. This lysozyme shares high sequence identity and is homologous to a previously identified lysozyme CpLYZ1 isolated fromC. plicataand with HcLyso3 isolated fromHyriopsis cumingii. The full-length cDNA of CpLYZ2 is 913 bp long, which includes an open reading frame (ORF) of 486 bp, a 3′ untranslated region (UTR) of 389 bp and a 5′ UTR of 38 bp. The ORF encodes a putative polypeptide of 161 amino acids with a predicted molecular mass of 18.2 kDa and a theoretical isoelectric point of 6.56. CpLYZ2 mRNA transcripts can be detected in hemocytes, hepatopancreas, muscle, gills and mantle tissues, the greatest expression being observed in the gills. CpLYZ2 expression in hemocytes, hepatopancreas and gills increased significantly after the mussel was challenged withAeromonas hydrophila. Furthermore, the optimal pH and temperature for enzyme activity of the recombinant CpLYZ2 were 5.5 and 50°C, respectively. The recombinant lysozyme protein exhibited bacteriolytic activity againstEscherichia coli,A. hydrophila,Staphyloccocus aureus,Bacillus subtilis,Streptococcussp. andStaphylococcus epidermidis. The findings of this study help to elucidate immune responses in molluscs and will thus expedite disease management of these key freshwater species, in turn boosting pearl culture in eastern Asia.

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