Two duplicated chicken-type lysozyme genes in disc abalone Haliotis discus discus: Molecular aspects in relevance to structure, genomic organization, mRNA expression and bacteriolytic function

• Two abalone c-type lysozymes (abLysCs) with conserved family features were identified.
• Coding region of both abLysCs were distributed within 5 exons and promoter regions of abLysCs were examined.
• Highest mRNAs in mantle (abLysC1) and hepatopancreas (abLysC2) suggested possible role in defense and digestion, respectively.
• Induced abLysC mRNA expression was detected against LPS and pathogen challenges in gill and hemocytes.
• Purified (r)abLysCs had differential bacteriolytic specificities with varying optimum lytic conditions.

Lysozymes are crucial antibacterial proteins that are associated with catalytic cleavage of peptidoglycan and subsequent bacteriolysis. The present study describes the identification of two lysozyme genes from disc abalone Haliotis discus discus and their characterization at sequence-, genomic-, transcriptional- and functional-levels. Two cDNAs and BAC clones bearing lysozyme genes were isolated from abalone transcriptome and BAC genomic libraries, respectively and sequences were determined. Corresponding deduced amino acid sequences harbored a chicken-type lysozyme (LysC) family profile and exhibited conserved characteristics of LysC family members including active residues (Glu and Asp) and GS(S/T)DYGIFQINS motif suggested that they are LysC counterparts in disc abalone and designated as abLysC1 and abLysC2. While abLysC1 represented the homolog recently reported in Ezo abalone [1], abLysC2 shared significant identity with LysC homologs. Unlike other vertebrate LysCs, coding sequence of abLysCs were distributed within five exons interrupted by four introns. Both abLysCs revealed a broader mRNA distribution with highest levels in mantle (abLysC1) and hepatopancreas (abLysC2) suggesting their likely main role in defense and digestion, respectively. Investigation of temporal transcriptional profiles post-LPS and -pathogen challenges revealed induced-responses of abLysCs in gills and hemocytes. The in vitro muramidase activity of purified recombinant (r) abLysCs proteins was evaluated, and findings indicated that they are active in acidic pH range (3.5–6.5) and over a broad temperature range (20–60 °C) and influenced by ionic strength. When the antibacterial spectra of (r)abLysCs were examined, they displayed differential activities against both Gram positive and Gram negative strains providing evidence for their involvement in bacteriolytic function in abalone physiology.

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