The Acheta domesticus Densovirus, Isolated from the European House Cricket, Has Evolved an Expression Strategy Unique among Parvoviruses

Abstract

Insect parvoviruses (densoviruses) belong to the Densovirinae subfamily of the Parvoviridae and are small, isometric, nonenveloped viruses (diameter, ϳ25 nm) that contain a linear single-stranded DNA of 4 to 6 kb (2, 3, 27). These viruses can be subdivided into two large groups, those with ambisense genomes and those with monosense genomes. Like vertebrate parvoviruses, all densoviruses have a genomic DNA with hairpins at both ends, often (but not necessarily for all genera) as inverted terminal repeats (ITRs). All densoviruses with ambisense genomes package both complementary strands in equimolecular ratios as single strands in separate capsids (27). The nonstructural (NS) gene cassette is found in the 5Ј half of one genome strand, and the structural protein (VP) gene cassette is found in the 5Ј half of the complementary strand. By convention, the genome is oriented so that the NS cassette is found in the left half. Expression strategies of densoviruses often involve (alternative) splicing and leaky scanning translation mechanisms (28). So far, the near-atomic structures of three densoviruses, Penaeus stylirostris densovirus (PstDNV), Bombyx mori densovirus 1 (BmDNV-1), and Galleria mellonella densovirus (GmDNV), have been solved (10,11,21). The capsid of densoviruses consists of 60 subunits (Tϭ1) of identical proteins that may contain N-terminal extensions not involved in capsid formation but that confer additional functions to the capsid. One of these functions is a phospholipase A2 (PLA2) activity that is required for genome delivery during infection (34). Densoviruses are usually highly pathogenic for their natural hosts (5).The monosense densoviruses have been classified into 3 uniform genera, i.e., Iteravirus, with a 5.0-kb genome, 0.25-kb ITRs, and a PLA2 motif in VP; Brevidensovirus, with a 4.0-kb genome, no ITRs but terminal hairpins, and no PLA2 motif; and Hepanvirus, with a single member, hepatopancreatic parvovirus, with a 6.3-kb genome also lacking a PLA2 motif and ITRs but with 0.2-kb terminal hairpins (23,27). In contrast, the ambisense densoviruses have been classified into one uniform genus, Densovirus, with a 6-kb genome and 0.55-kb ITRs, and a second genus, Pefudensovirus, with only Periplaneta fuliginosa densovirus (PfDNV) as a member, with a 5.5-kb genome, 0.2-kb ITRs, and a split VP gene cassette (2, 26). Ribosome frameshifts have been proposed to connect its VP open reading frames (ORFs) (33). So far, all ambisense densoviruses have an N-terminal PLA2 motif in their largest VP. Some sequenced ambisense densoviruses, e.g., Myzus persicae densovirus (MpDNV) (32), Blattella germanica densovirus (BgDNV) (18), and Planococcus citri densovirus (PcDNV) (25), are as yet unclassified. The ambisense virus Culex pipiens densovirus (CpDNV) has a different genome organization for both the NS

Expression Strategy of Aedes albopictus Densovirus

“…PCR-amplified products (primers in Table 1) were transcribed in vitro with [␣-32 P]UTP and T7 RNA polymerase (NEB) (13). Northern blots using 10 g of total RNA (13,14) revealed transcript sizes of 3.4 and 1.3 kb with the VP-specific probe and a 3.4-kb transcript with the NS-specific probe (Fig. 1A).…”

The Structure and Host Entry of an Invertebrate Parvovirus

“…Densoviruses are highly pathogenic, and those that use insect hosts usually kill 90% of the larvae within a few days (2). Densoviruses pose a threat to commercial invertebrates such as shrimp (3), silkworms (4), and crickets (5,6). Some highly pathogenic densoviruses are potential selective pesticides for vectors that transmit mosquito-borne diseases (7).…”

Gene Expression of Five Different Iteradensoviruses: Bombyx mori Densovirus, Casphalia extranea Densovirus, Papilio polyxenes Densovirus, Sibine fusca Densovirus, and Danaus plexippus Densovirus

“…So far, VP transcripts of different densoviruses with one VP ORF are not spliced but sets of N-terminally extended proteins are generated by leaky scanning (6,(21)(22)(23). Among others, densoviruses from Blattella germanica (BgDV) and Acheta domestica (AdDV) have two VP ORFs that are joined by splicing and generate proteins with alternate N termini in addition to nested N-terminally extended sets by leaky scanning (4,24). Leaky scanning is also important for the NS proteins of densoviruses from different genera (4,6,(21)(22)(23)(24).…”

“…Among others, densoviruses from Blattella germanica (BgDV) and Acheta domestica (AdDV) have two VP ORFs that are joined by splicing and generate proteins with alternate N termini in addition to nested N-terminally extended sets by leaky scanning (4,24). Leaky scanning is also important for the NS proteins of densoviruses from different genera (4,6,(21)(22)(23)(24). However, splicing determines whether NS3 or NS1/NS2 is expressed for the ambisense densoviruses, whereas the two NS genes of the brevidensoviruses have overlapping promoters so that transcripts start at either side of the NS1 start codon, allowing either NS1 or the downstream NS2 to be expressed (21)(22)(23)(24).…”