Hairpin Transfer-Independent Parvovirus DNA Replication Produces Infectious Virus.

Parvoviruses package a linear single-stranded DNA genome with hairpin structures at both ends. It has been thought that terminal hairpin sequences are indispensable for viral DNA replication. Here, we provide evidence that the hairpin-deleted duplex genomes of human bocavirus 1 (HBoV1) replicate in human embryonic kidney 293 (HEK293) cells. We propose an alternative model for HBoV1 DNA replication in which the leading strand can initiate strand displacement without hairpin transfer. The transfection of the HBoV1 duplex genomes that retain a minimal replication origin at the right end (OriR) but with extensive deletions in the right-end hairpin (REH) generated viruses in HEK293 cells at a level 10 to 20 times lower than that of the wild-type (WT) duplex genome. Importantly, these viruses that have a genome with various deletions after the OriR but not the one retaining only the OriR replicated in polarized human airway epithelia. We discovered that the 18-nucleotide (nt) sequence (nt 5403 to 5420) beyond the OriR was sufficient to confer virus replication in polarized human airway epithelia, although its progeny virus production was ∼5 times lower than that of the WT virus. Thus, our study demonstrates that hairpin transfer-independent productive parvovirus DNA replication can occur. IMPORTANCE Hairpin transfer-independent parvovirus replication was modeled with human bocavirus 1 (HBoV1) duplex genomes whose 5' hairpin structure was ablated by various deletions. In HEK293 cells, these duplex viral genomes with ablated 5' hairpin sequence replicated efficiently and generated viruses that productively infected polarized human airway epithelium. Thus, for the first time, we reveal a previously unknown phenomenon that productive parvovirus DNA replication does not depend on the hairpin sequence at REH to initiate rolling-hairpin DNA replication. Notably, the intermediates of viral DNA replication, as revealed by two-dimensional electrophoresis, from transfections of hairpin sequence-deleted duplex genome and full-length genome in HEK293 cells as well as from virus infection of polarized human airway epithelia are similar. Thus, the establishment of the hairpin transfer-independent parvoviral DNA replication deepens our understanding of viral DNA replication and may have implications in the development of parvovirus-based viral vectors with alternative properties.