Newly identified C‐type lysozyme in Chinese soft‐shelled turtle (Pelodiscus sinensis) exhibits potent antimicrobial activity

Lysozymes play vital roles in humoural immune response against bacterial invasion by its lytic activity. In the present study, a new C-type lysozyme was identified and characterized from Chinese soft-shelled turtle Pelodiscus sinensis. The full-length cDNA of PslysC was of 923 bp, encoding a polypeptide of 148 amino acid residues. The multiple alignments and phylogenetic relationship analysis revealed the highly enzyme-related conserved residues. The real-time PCR analysis suggested that PslysC was constitutively expressed in a wide range of tissues with highest level in blood cells and liver. The expression of PslysC could be significantly up-regulated under Aeromonas jandaei infection and ammonia exposure, while no significant changes were found under Poly I:C infection. The rPslysC protein was expressed in E. coli and purified by Ni-NTA. The optimal pH and temperature for rPslysC protein lytic activities were determined at pH 7 and 30℃. rPslysC can inhibit the growth of eight kinds of Gram-negative bacteria, and three kinds of Gram-positive bacteria. The binding activity of rPslysC to different microbial polysaccharides and microorganism was analysed. The results showed that rPslysC could bind to selected bacteria, and exhibit a strong binding activity to lipopolysaccharide and peptidoglycan, but a weak binding activity to β-glucan. This suggests that the binding activity might be the major mechanism of action to realize the antibacterial activity. The present study will provide helpful evidence to further understand the innate immunity of P. sinensis, and the interaction mechanisms of C-type lysozymes with bacterial membranes.