Tilapia is one of the important species in aquaculture in the world.In recent years,tilapia aquaculture has been badly damaged by streptococcusis.Streptococcus agalactiae is one of the major causative agents,which has caused severe economic losses in many species of freshwater,marine and estuarine fish worldwide.Vaccine is one of the main development future strategies to prevent fish streptococcusis.Some surface proteins of S.agalactiae have been documented and proved to be the ideal antigen candidates of protein vaccines,which included LrrG protein.The LrrG protein is one of the conserved surface proteins of S.agalactiae,which exists in all kinds of serotype of S.agalactiae strains.The LrrG protein of S.agalactiae isolated from human has been verified to protect mice from S.agalactiae infection.However,it has not been reported whether the LrrG protein from fish S.agalactiae isolates possessed the similar effect.In order to obtain plenty of LrrG protein and explore its immune protection in tilapia,in this study we designed the specific primers and cloned the LrrG gene sequence from tilapia virulent S.agalactiae strain according to the reported LrrG gene in GenBank of human S.agalactiae.The sequencing results showed that the ORF of the cloned LrrG was 2 361 bp in length and encoded 786 amino acids and contained 3 conserved LRR(Leucine-rich repeat)domains.The colis accounted for 68.83% in its secondary structure.NCBI Blast indicated that the homology of LrrG gene sequences of S.agalactiae from tilapia and human was 98.48%.The LrrG protein could form antigenic epitope based on the analysis of three indexes,i.e.hydrophilicity,surface probability and antigenic index.This suggests that the LrrG protein may have potential immunogenicity.The LrrG fragment was subcloned into pET-32a(+)vector and expressed in E.coli BL21(DE3)by IPTG induction at 22 ℃.SDS-PAGE showed the molecular weight of expressed protein was 108.9 ku,which is equal to the expected protein size.The recombinant protein existed as soluble protein and inclusion bodies.The soluble protein was highly purified by Ni-chelating affinity chromatography and the concentration could reach 3.40 mg/mL by ultrafiltration.Preliminary experiment showed that the relative percent survival of tilapia immunized with LrrG protein against tilapia streptococcicosis was 69.28%.ELISA analysis indicated the serum antibody titer of post-immuned tilapia for 4 weeks was 1:800.These findings provide the useful information for further investigation to evaluate the potential value of the LrrG protein as tilapia genetic engineering vaccine.