Streptococcus agalactiae surface proteins can stimulate the host to produce protective immune.The protein C5a peptidase is widespread in a variety of serotypes of Streptococcus strains,highly conserved and has good immunogenicity.It is a possible carrier protein that could induce protective immune response by itself.However,the immune epitopes and the role of virulence groups of S.agalactiae scpB gene of fish is still unclear.In this study,the scpB gene was amplified from genome DNA of S.agalactiae isolated from tilapia by PCR with specific primers.Restriction analysis and DNA sequencing confirmed that the scpB gene has a ORF of 3 405 bp,encoding 1 134 amino acid,which was highly conserved and had a surprising degree of homology among strains isolated from other mammals.Molecular analysis of scpB gene was performed by bioinformatics tools such as DNAstar,Clustal X 2.0,MEGA 5.05,ExPASy ProtParam,NCBI Protein blast,and NCBI Conserved Domain et al.The results showed that the amino acid encoded by the scpB gene contained 3 catalytic triad,7 putative active sites,2 specific hits,and 4 super families (2 Peptidases-S8-S53，1 DUF1034,and 1 FlgD-lg).Moreover,it was found that C5a peptidase had mutiple epitopes,suggesting that the protein may have strong immunogenicity.Then the PCR product was inserted into pET32a(+) and the constructed recombinant plasmid pET32a(+)/scpB was transformed to E.coli BL21(DE3) for expression.The positive colony,which were identified by PCR and digestion (EcoRⅠand XhoⅠ),showed that the recombinant plasmid pET32a (+)/scpB was constructed successfully.This study paved the way for further study of immune mechanism and polypeptide vaccine based on C5a peptidase from the tilapia S.agalactiae.