Molecular characterization and expression analysis of a complement component C3 in large yellow croaker (Larimichthys crocea)

• Molecular cloning of complete L.c-C3 cDNA were firstly conducted.
• Validity of the genome and complete cDNA conservative were characterized.
• RT-qPCR assays showed the highest expression level in liver.
• No significant difference was found during different development period.
• L.c-C3 mRNA expression quantity increased obviously under bacterial infection.

The complement system has been discovered in invertebrates and vertebrates, and plays a crucial role in the innate defense against common pathogens. Complement component 3 is a key molecule in the complement system, whose activation is essential for all the important functions performed by this system. In this study, the complete C3 cDNA sequence was isolated from the large yellow croaker (Larimichthys crocea), which was high similarity to other complement C3. We reported the primary sequence, tissue expression profile, polypeptide domain architecture and phylogenetic analysis of L. crocea complement component C3 (L.c-C3) gene. Its open reading frame (ORF) is 4962 bp and encodes for 1653 amino acids with a putative signal peptide of 23 amino acid residues. The deduced amino acid sequence showed that L.c-C3 has conserved residues and domains known to be crucial for C3 function. Phylogenetic analysis showed that L. crocea was closely related to Miichthys miiuy. The mRNA expressions of L.c-C3 was detectable at different tissues. L.c-C3 was expressed in a wide range of adult tissues, it showed highest expression in the liver. But the different developmental stages from fertilized egg to newborn larvae of the large yellow croaker the highest expression levels of L.c-C3 gene were not found. Bacterial challenge experiments showed that the levels of L.c-C3 mRNA expression were up-regulated in the liver, spleen and brain of adult large yellow croaker respectively. The results showed that L.c-C3 mRNA expression in the large yellow croaker is influenced by bacterial stress and L.c-C3 might play an important role in immunity mechanisms. This study will further increase our understanding of the function of L.c-C3 and molecular mechanism of innate immunity in teleosts.