Abstract: The purpose of this study was to prepare polyclonal antibody against early pregnancy factor (EPF) of yak and lay the foundation for developing an efficient and rapid diagnosis of early pregnancy for yak in clinical field. EPF gene was amplified by RT-PCR using RNA extracted from yak placenta and ovarian tissue and reverse transcribed as template. Then the EPF gene was subcloned into the modified vector pET28-His10-Sumo to construct the pET28-His10-Sumo-EPF recombinant plasmid.After PCR and sequencing identification,the positive recombinant plasmid was transformed into BL21(DE3) for expression.IPTG was used to induce and optimize the reaction conditions.The recombinant EPF protein was purified and immunized mice to produce polycolonal antibodies. The results showed that, after PCR amplification,the target was about 300 bp,which was consistent with the expected results, sequencing identification confirmed the recombinant positive plasmid was successfully constructed.After EPF protein was expressed in E. coli, the protein molecular weight was 29 ku (includes target proteins and SUMO tagged proteins) using IPTG inducer.The protein expression level reached the highest at 37℃ than other controls and induced concentration of IPTG was 300 nmol·L^-1 for 6 h.Western blot results showed that the protein was expressed in both supernatant and inclusion body, and the mouse immunized antiserum could react with the purified recombinant EPF protein. The recombinant EPF protein was successfully expressed by prokaryotic expression system,and the mouse anti-EPF polyclonal antibodies with good immunogenicity was prepared.

Key words: yak, early pregnancy factor (EPF), prokaryotic expression, polyclonal antibody