Identification and expression analysis of the g-type and c-type lysozymes in grass carp Ctenopharyngodon idellus

Lysozyme is an important molecule of innate immune system for the defense against bacterial infections. We identified two genes encoding g-type and c-type lysozymes from grass carp Ctenopharyngodon idellus by the RACE method. The deduced amino acids of both lysozymes possessed typical structural residues and conserved catalytic sites similar to their counterparts across the species. In contrast, there was only 8.6% similarity of amino acid sequence between these two lysozymes. Phylogenetic analyses revealed that these two genes evolved at different rate. C-type lysozyme of grass carp was diverged early in the evolutionary history. Moreover, the expression patterns of these two genes differed. The mRNA levels of both genes were increased after bacterial infection, but the up-regulation of g-type lysozyme was much stronger than that of c-type lysozyme. We also showed that the c-type and g-type recombinant lysozymes possessed different lytic activities against fish bacterial pathogens. These results confirmed that both lysozymes play important roles in the defense of grass carp against bacterial infections. The g-type lysozyme may be induced for the defense against bacterial infections, while c-type lysozyme might be the main molecule for the house-keeping defense under normal conditions. These two types of lysozymes likely use different mechanisms to regulate their expressions.