Evidence for internal initiation of RNA synthesis by the Hepatitis C virus RNA-dependent RNA polymerase NS5B in cellulo.

Schult, Philipp; Nattermann, Maren; Lauber, Chris; Seitz, Stefan; Lohmann, Volker

Initiation of RNA synthesis by the Hepatitis C virus (HCV) RNA-dependent-RNA-polymerase (RdRp) NS5B has been extensively studied in vitro and in cellulo Intracellular replication is thought to rely exclusively on terminal de novo initiation, as it conserves all genetic information of the genome. In vitro, however, additional modes of initiation have been observed. In this study, we aimed to clarify whether the intracellular environment allows for internal initiation of RNA replication by the HCV replicase. We used a dual luciferase replicon harboring a terminal and an internal copy of the viral genomic 5' UTR, which was anticipated to support non-canonical initiation. Indeed, a shorter RNA species was detected by northern blotting with low frequency, depending on the length and sequence composition upstream of the internal initiation site. By introducing mutations at either site, we furthermore established that internal and terminal initiation shared identical sequence requirements. Importantly, lethal point mutations at the terminal site resulted exclusively in truncated replicons. In contrast, the same mutations at the internal site abrogated internal initiation, suggesting a competitive selection of initiation sites, rather than recombination or template-switching events. In conclusion, our data indicate that the HCV replicase is capable of internal initiation in its natural environment, although functional replication likely requires only terminal initiation. Since many other positive strand RNA viruses generate subgenomic messenger RNAs during their replication cycle, we surmise that the capability for internal initiation it is a common and conserved feature of viral RdRps.ImportanceMany aspects of viral RNA replication of Hepatitis C virus (HCV) are still poorly understood. The process of RNA synthesis is driven by the RNA-dependent RNA-polymerase NS5B. Most mechanistic studies on NS5B so far were performed with in vitro systems using isolated recombinant polymerase. In this study we present a replicon model, which allows the intracellular assessment of non-canonical modes of initiation by the full HCV replicase. Our results add to the understanding of the biochemical processes underlying initiation of RNA synthesis by NS5B by the discovery of internal initiation in cellulo Moreover, they validate observations made in vitro, showing that the viral polymerase acts very similarly in isolation and in complex with other viral and host proteins. Finally, these observations provide clues about the evolution of RdRps of positive strand RNA viruses, which might contain the intrinsic ability to initiate internally.