A Tyr residue in the reverse transcriptase domain can mimic the protein-priming Tyr residue in the terminal protein domain of a hepadnavirus P protein.

Hepadnaviruses are the only known viruses that replicate by protein-primed reverse transcription. Beyond the conserved reverse transcriptase (RT) and RNase H domains, their polymerases (P proteins) carry a unique terminal protein (TP) domain that provides a specific Tyr residue, Tyr96 in duck hepatitis B virus (DHBV), to which the first nucleotide of minus-strand DNA is autocatalytically attached and extended by three more nucleotides. In vitro reconstitution of this priming reaction with DHBV P protein and cellular chaperones had revealed strict requirements for the Dε RNA stem-loop as a template and for catalytic activity of the RT domain plus RNA-binding competence of the TP domain. Chaperone dependence can be obviated by using a truncated P protein (miniP). Here, we found that miniP with a tobacco etch virus (TEV) protease cleavage site between TP and RT (miniP(TEV)) displayed authentic priming activity when supplied with α-(32)P-labeled deoxynucleoside triphosphates; however, protease cleavage revealed, surprisingly, that the RT domain was also labeled. RT labeling had identical requirements as priming at Tyr96 and originated from dNMP transfer to a unique Tyr residue identified as Tyr561 in the presumed RT primer grip motif. Mutating Tyr561 did not affect Tyr96 priming in vitro and only modestly reduced replication competence of an intact DHBV genome; hence, deoxynucleotidylated Tyr561 is not an obligate intermediate in TP priming. However, as a first alternative substrate for the exquisitely complex protein-priming reaction, dNMP transfer to Tyr561 is a novel tool to further clarify the mechanism of hepadnaviral replication initiation and suggests that specific priming inhibitors can be found.