AAV2 can replicate its DNA by a rolling hairpin or rolling circle mechanism, depending on the helper virus

Adeno-associated virus type 2 (AAV2) is a small, non-pathogenic, helper virus-dependent parvovirus with a single-stranded (ss) DNA genome of approximately 4.7 kb. AAV2 DNA replication requires the presence of a helper virus such as adenovirus type 5 (AdV5) or herpes simplex virus type 1 (HSV-1) and is generally assumed to occur as a strand-displacement rolling hairpin (RHR) mechanism initiated at the AAV2 3-prime inverted terminal repeat (ITR). We have recently shown that AAV2 replication supported by HSV-1 leads to the formation of double-stranded head-to-tail concatemers, which provides evidence for a rolling circle replication (RCR) mechanism. We have revisited AAV2 DNA replication and specifically compared the formation of AAV2 replication intermediates in presence of either HSV-1 or AdV5 as the helper virus. The results confirmed that the AAV2 DNA replication mechanism is helper virus-dependent and follows a strand-displacement RHR mechanism when AdV5 is the helper virus and primarily an RCR mechanism when HSV-1 is the helper virus. We also demonstrate that recombination plays a negligible role in AAV2 genome replication. Interestingly, the formation of high molecular weight AAV2 DNA concatemers in presence of HSV-1 as the helper virus was dependent on an intact HSV-1 DNA polymerase.