The 3D protein of duck hepatitis A virus type 1 binds to a viral genomic 3′ UTR and shows RNA-dependent RNA polymerase activity

To explore the RNA-dependent RNA polymerase (RdRP) function of the 3D protein of duck hepatitis A virus type 1 (DHAV-1), the gene was cloned into the pET-32a(+) vector for prokaryotic expression. The 3′ untranslated region (3′ UTR) of DHAV-1 together with a T7 promoter was cloned into the pMD19-T vector for in vitro transcription of 3′ UTR RNA, which was further used as a template in RNA-dependent RNA polymerization. In this study, three methods were applied to analyze the RdRP function of the 3D protein: (1) ammonium molybdate spectrophotometry to detect pyrophosphate produced during polymerization; (2) quantitative reverse transcription PCR (RT-qPCR) to investigate the changes in RNA quantity during polymerization; and (3) electrophoresis mobility shift assay to examine the interaction between the 3D protein and 3′ UTR. The results showed the 3D protein was successfully expressed in bacteria culture supernatant in a soluble form, which could be purified by affinity chromatography. In 3D enzymatic activity assays, pyrophosphate and RNA were produced, the amounts of which increased based on approximative kinetics, and binding of the 3D protein to the 3′ UTR was observed. These results indicate that prokaryotically expressed soluble DHAV-13D protein can bind to a viral genomic 3′ UTR and exhibit RdRP activity.