REVERSE TRANSCRIPTION IN HEPATITIS-B VIRUSES IS PRIMED BY A TYROSINE RESIDUE OF THE POLYMERASE

All known DNA polymerases require primers for the initiation of DNA sy nthesis. While cellular polymerases and reverse transcriptases use fre e hydroxyl groups of RNA or DNA, the DNA polymerases of certain animal viruses and bacteriophages depend upon hydroxyl groups of amino acid residues within proteins as primers for DNA synthesis. Recently, the r everse transcriptase of a hepadnavirus has been shown to prime RNA-dir ected DNA synthesis from an internal site of the polypeptide (G. H. Wa ng and C. Seeger, Cell 71:663-670, 1992). In this report we demonstrat e that a tyrosine residue of the polymerase polypeptide is the site of a phosphodiester linkage with the first nucleotide of minus-strand DN A. This tyrosine residue is located within an aminoterminal domain of the polymerase polypeptide and is indispensable for the priming of rev erse transcription. Our results demonstrate that the hepatitis B virus reverse transcriptase can initiate DNA synthesis without the requirem ent for tRNA as a primer.