A recombinant baculovirus was constructed containing a copy of the hepatitis B virus (HBV) genome which was inserted to produce an in-frame fusion of the precore (pre-C) coding region with the first 11 amino acids of the polyhedrin gene. The recombinant baculovirus expressed the 25K pre-C protein and two novel proteins, of approximately 93K and 72K. Both the 93K and 72K proteins are recognized by an anti-polymerase monoclonal antibody. Northern blot analysis of the mRNA produced during infection of cells by the HBV recombinant baculovirus detected only one HBV mRNA species, suggesting that the three HBV-specific proteins expressed are translated from the same mRNA. No larger fusion proteins cross-reacting with either anti-core or polymerase antibodies were detected. These findings suggest that the two proteins encoded within the HBV polymerase gene are not produced via a core-polymerase fusion intermediate but by internal binding of ribosomes. These results are the first clear demonstration of efficient expression of two bona fide unprocessed polymerase proteins in a 1:1 ratio from an unspliced pre-C mRNA-like transcript. With the successful expression of the polymerase gene in insect cells it is now possible to produce large amounts of these proteins, allowing a more detailed structural and functional analysis of these proteins.


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