Evidence for involvement of a ribosomal leaky scanning mechanism in the translation of the hepatitis B virus pol gene from the viral pregenome RNA.

In retroviruses, the pol gene is expressed in the form of a gag-pol fusion protein by the mechanism of ribosomal frameshifting. In studies of the possible mechanism of hepadnaviral pol protein synthesis, recent results have ruled out core-pol fusion protein synthesis by ribosomal frameshifting. In this study, an in vitro transcription and translation coupling system was used to demonstrate that the HBV core and pol proteins could be synthesized independently using the pregenome RNA template. The result has led us to design experiments to distinguish between the involvement of a termination-reinitiation, internal initiation, or leaky scanning mechanism in the pol protein synthesis. In vitro experiments were then carried out to measure the amount of pol proteins being synthesized from (i) the preC mRNA, which contained an extra AUG and seven more nucleotides at the 5'-end in comparison with the pregenome RNA; (ii) the pregenome RNA in the presence of various amounts of antisense RNA annealing to the 5'-end of the pregenome RNA; and (iii) the pregenome RNA with an additional hairpin structure located upstream of the C gene. Results indicated that the synthesis of both core and pol proteins was concomitantly reduced in these three conditions, which suggested that leaky scanning is the most probable mechanism for pol protein synthesis in vitro. To further verify the mechanism in vivo, experiments were performed to assay the activity of DNA polymerase in virions, which were obtained from hepatoma cells transfected by plasmids containing either a wild-type sequence (5'-GGCATGG-3') or an optimal initiation context (5'-ACCATGG-3') of the C gene. Transfection results showed that the plasmid-containing mutations of the C gene significantly decreased the DNA polymerase activity in virions. This observation supports our hypothesis that the leaky scanning model is involved in the synthesis of pol protein.