characterization and phylogenetic analysisof a g-type lysozyme gene variant from the skin mucus of grass carp (ctenopharyngodon idella)

The g-type lysozyme is one of the three major diverse lysozyme types recognized in the animal kingdom including fish. using the rt-pcr technique, a 555 bp cdna fragment encoding a g-type lysozymewas isolated from the skin mucus of ctenopharyngodon idella using homolog primes. the cdna named ci-kh, codes for 185 amino acids with a predicted molecular weight of 20.49 kda and theoretical pi of 9.13. the sequence consists of one cysteine residue with no predicted signal peptide. domain analysis showed e-value of 3.74e-107 with the conserved domain of lysozyme-like superfamily (cd01021) between amino acid residues 12 to 184. multiple alignment with the lysozyme genes from other fish species revealed that this protein have a goose egg white lysozyme (gewl) domain containing two conserved catalytic residues (glu73 and asp97) and n-acetyl-d-glucosamine binding site (glu73, asp85, asp97, tyr100, his101, his102, ile119, tyr147, asn148). protein structure prediction software revealed a prediction of 58% and 42% of a-helical and random coils for the coding sequence of ci-kh, respectively.the 3d model of ci-kh revealed that this protein was mainly composed of five main helices and random coils. phylogenetic analysis indicated that ci-kh matched the group of five grass carp g-type lysozyme transcript variants with 86-99% similarity. among the five variants of this gene, ci-kh sequence had the highest and lowest genetic distance with c. idella variant x2 (8.6%) and x3 (1.1%) sequences, respectively. we conclude that ci-kh as a different variant of g-type lysozyme cannot be ruled out.