• endonuclease
Terminal sequences of RNA virus genomes have important regulatory functions in transcription and replication. The genome termini of negative-strand RNA viruses (NSV) are composed of inverted terminal repeats (ITRs), which represent promoter regions that control the asymmetric expression of genome (vRNA) and antigenome (cRNA). Replication usually initiates with a single nucleotide at the 3′ terminus of the template strand and in some NSV requires the direct interaction of the ITRs (1–3). Efficient amplification of viral genomic RNA, therefore, critically depends on the synthesis and maintenance of correct terminal sequences. Protection of terminal integrity is of particular relevance for RNA viruses that persist in vivo, because terminal genome deterioration during persistent infection has been described for several RNAviruses from distinct virus orders and families (4). Furthermore, 5′ triphosphate groups were identified as pathogen-associated molecular patterns that strongly contribute to the recognition of NSV by the innate immune system (5–7). It is therefore not surprising that many NSV developed sophisticated mechanisms to modify the terminal structures of their genomes. Segmented NSV of the Arena-, Bunya-, andOrthomyxovirus families use prime-and-realign mechanisms in which short, internally synthesized primers realign with the 3′ end of the template strand to initiate genome replication (8–11). This process regenerates the 5′ termini from internal templates during each round of replication and generates a triphosphorylated 5′ nucleotide without template function that can be removed by a nuclease activity of the viral polymerase (10).
Borna disease virus (BDV) has a nonsegmented RNA genome of negative polarity (12). Replication and transcription of the BDV genome occurs in the nucleus and is not associated with overt cytotoxicity (13), enabling the virus to establish long-term persistence in a variety of cultured cells and animals. The termini of BDV v- and cRNA molecules remain stable even after prolonged persistence and display unique characteristics, indicating that they are modified during the replication process. For example, we found that both molecules possess four nucleotides at their 3′ terminus that lack an apparent template at the 5′ end of the opposite strand (14). We initially proposed that the 5′ termini are generated by preferential internal initiation of replication on rare v- and cRNA molecules, providing the purportedly missing nucleotides at the 5′ terminus (14). However, this model has been challenged by the fact that all attempts to demonstrate the existence of these rare v- and cRNA molecules have failed. Furthermore, we recently found that the 5′ termini of the v- and cRNA molecules are mono- rather than triphosphorylated (7), indicating that enzymatic modification rather than simple internal initiation is responsible for the recessed 5′ termini. To explore alternative possibilities for the modification of the BDV termini, we generated a panel of recombinant viruses from cDNAs with distinct terminal mutations. Analyses of the terminal sequences of the recovered viruses revealed that the recessed 5′ terminus of the BDV genome is not a consequence of internal initiation of replication; rather, it is a consequence of elongation of the 3′ termini of v- and cRNA molecules, which occurs after the template strand has been copied. A picture emerges, which shows that the 3′ termini of the copied v- and cRNA molecules realign to internal sequence motifs on the newly synthesized strand that provide the template for the synthesis of the missing nucleotides.
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Results
3′ Termini of BDV v- and cRNA Are Not Synthesized from Standard Templates.
We previously showed that the genomic 3′ overhangs of BDV are maintained, even if BDV is recovered from an antigenome providing the purportedly missing nucleotides at the 5′ terminus (pBDVc) (Fig. 1A) (14). To challenge the internal initiation model, we modified pBDVc to encode altered terminal sequence motifs (pBDVc-CG) (Fig. 1A), or introduced terminal deletions of four nucleotides eliminating the sequence information for the v- and cRNA 3′ overhangs (pBDV) (Fig. 1A) or of five nucleotides removing, in addition to the sequence information for the 3′ overhangs, a single nucleotide from each end of the antigenome (pBDV-ΔGC) (Fig. 1A). Remarkably, constructs pBDVc-CG and pBDV were efficiently rescued, whereas construct pBDV-ΔGC failed to support the recovery of a recombinant virus in repeated attempts (Fig. 1B). Furthermore, the growth properties of BDVc-GC and BDV were indistinguishable from those of BDVc (Fig. 1B), indicating that pBDVc-CG and pBDV encoded fully functional BDV antigenomes.