TRANSLATION OF THE HEPATITIS-B VIRUS P-GENE BY RIBOSOMAL SCANNING AS AN ALTERNATIVE TO INTERNAL INITIATION

The hepatitis B virus (HBV) P gene which encodes the reverse transcrip tase and other proteins required for replication is expressed on the b icistronic mRNA pregenome which also encodes the capsid protein in its first cistron. Recent results have suggested that the hepadnaviral P gene is translated by internal entry of ribosomes upstream from the P gene, in the overlapping C gene. Using a reporter gene fused to the HB V C or P gene, we demonstrate that the C sequence does not allow inter nal initiation of translation. Alternatively, our results support a mo del in which the HBV P gene is translated by ribosomes which scan from the capped extremity of the bicistronic mRNA pregenome. The mechanism by which the ribosomes scan past four AUGs before they initiate trans lation at the P AUG was analyzed. Our results show that these AUGs are skipped via two mechanisms: leaky scanning on AUGs in a weak or subop timal initiation context and translation of an out-of-C-frame minicist ron followed by reinitiation at P AUG. The minicistron translation all ows ribosomes to bypass an AUG in a favorable context that would other wise be used as a start codon for translation of a truncated capsid pr otein. Our results suggest that this elaborated scanning mechanism per mits the coordinate expression of the HBV C and P genes on the viral b icistronic mRNA pregenome.