Kinetics of Synthesis and Turnover of the Duck Hepatitis B Virus Reverse Transcriptase

Hepadnaviral reverse transcription occurs in subviral capsids in which the core protein surrounds the reverse transcriptase (“polymerase”) and the pregenomic RNA. The pregenomic RNA is the template for reverse transcription and also the bicistronic mRNA for core and polymerase. The pregenomic RNA structure and the capsid stoichiometry imply that vastly more core would be translated than polymerase. Previously, we found that duck hepatitis B virus polymerase unexpectedly accumulates in the cytoplasm (Yao, E., Gong, Y., Chen, N., and Tavis, J. E. (2000) J. Virol.74, 8648–8657). The production mechanism and function of the excess polymerase are unknown. Here, we determined the kinetics of expression and degradation of polymerase and core in cells producing virus. Polymerase was translated 10% as rapidly as core, the half-life of nonencapsidated polymerase was very short, core had a very long half-life, and very few polymerase molecules were encapsidated. The presence of excess polymerase indicates that the translation rate of the polymerase is not limiting for encapsidation. Therefore, encapsidation must be regulated by other events, most likely binding of the polymerase to the pregenomic RNA. These data support the hypothesis that polymerase may have functions beyond copying the viral genome by demonstrating that the polymerase is a cytoplasmic protein that is only rarely encapsidated.