Detection of replicative intermediate with closed terminus of Bombyx densonucleosis virus

Two kinds of Bombyx densonucleosis virus (BmDNV), BmDNV-1 and 2, have been isolated from sericultural farms in Japan or China. These viruses are classified into the family Parvoviridae because of the small spherical virus particle containing a single-stranded linear DNA genome. Recent studies on the genome structure of these viruses suggested that BmDNV-2 was a new type of virus with unique replication mechanism, though that of BmDNV-1 was similar to parvoviruses. However, details about the replication mechanism of BmDNVs have not been reported so far. Here, in order to elucidate the difference on replication mechanism between BmDNVs and parvoviruses, we analyzed the structure of the replicative intermediate (RI) of BmDNV DNAs by PCR using specific primers designed for detection of RI with closed terminal structure (RI-CT) which is expected to be formed by replication with self-priming mechanism. PCR using the DNA from the cells infected with BmDNV-1 could detect the expected DNA fragment, showing the existence of RI-CT. On the other hand, no fragment could be amplified from the virion DNA of the BmDNVs and the DNA extracted from BmDNV-2-infected cells, respectively. These observations strongly suggested that the BmDNV-1 replicates with the "self-priming and hairpin-transfer" mechanism similar to the human parvoviruses, while BmDNV-2 does not.