Molecular characterisation of a goose-type lysozyme gene in Atlantic cod (Gadus morhua L.)

Lysozymes are antibacterial enzymes important in the innate immune defense of several animal phyla. An Atlantic cod goose-type (g-type) lysozyme EST was identified in a suppression subtractive hybridisation (SSH) cDNA library and the full-length cDNA (codg1) was obtained by RACE-PCR. The lysozyme gene is organised in five exons and four introns similar to g-type lysozyme genes in other fish species. Two different cod lysozyme transcripts, named codg1 and codg2, seem to be produced by the use of alternative transcription start sites (TSS) in the lysozyme gene. The alternative TSS cause a different exon I usage where exon Ia transcripts possess a putative signal peptide (codg1) while exon Ib transcripts (codg2) lack this feature. Lysozyme without the signal peptide was produced recombinantly in Escherichia coli and displayed muramidase activity against Micrococcus luteus cells at an unusually low pH. Gene expression analysis of codg1 and codg2 showed that both were expressed in several tissues with highest expression in the head kidney, peritoneum and spleen. Codg1 and codg2 were differentially expressed in some tissues. In the non-immunised control group, codg2 was expressed significantly higher in the head kidney compared to codg1, while an opposite expression profile was observed in the gills. Compared to non-immunised fish, a significant up-regulation of codg2 transcripts was observed in the peritoneum and gills after injection of formalin inactivated Listonella anguillarum indicating a role for g-type lysozyme in the innate defense system of cod.