Mechanism of translation of the hepadnaviral polymerase (P) gene.

Unlike many other reverse transcriptase genes, the polymerase (P) gene of the hepatitis B viruses is expressed by translational initiation from its own AUG codon rather than by ribosomal frameshifting during translation of the overlapping core gene (C). To explore the mechanism of its translation, we have fused the P gene of duck hepatitis B virus to the bacterial lacZ gene at a point 3' to the C-P overlap; this allows detection of the products of P translation with antisera to the lacZ-encoded protein. The C and P/Z coding regions were cloned downstream of a heterologous promoter and expressed in COS-7 cells. A single, bicistronic mRNA containing both C and P sequences is detected in these cells, and translational initiation occurs efficiently at the internally situated P AUG. Mutations affecting C translation have only minimal effects on P expression, in contrast to what would be expected from a modified scanning model for translation. We conclude that P translation depends on a mechanism other than scanning to allow internal entry of ribosomes to the region of the P initiator.