Expression strategy of Aedes albopictus densovirus

Bipartite genome and structural organization of the parvovirus Acheta domesticus segmented densovirus

Parvoviruses (family Parvoviridae) are currently defined by a linear monopartite ssDNA genome, T = 1 icosahedral capsids, and distinct structural (VP) and non-structural (NS) protein expression cassettes within their genome. We report the discovery of a parvovirus with a bipartite genome, Acheta domesticus segmented densovirus (AdSDV), isolated from house crickets (Acheta domesticus), in which it is pathogenic. We found that the AdSDV harbors its NS and VP cassettes on two separate genome segments. Its vp segment acquired a phospholipase A2-encoding gene, vpORF3, via inter-subfamily recombination, coding for a non-structural protein. We showed that the AdSDV evolved a highly complex transcription profile in response to its multipartite replication strategy compared to its monopartite ancestors. Our structural and molecular examinations revealed that the AdSDV packages one genome segment per particle. The cryo-EM structures of two empty- and one full-capsid population (3.3, 3.1 and 2.3 Å resolution) reveal a genome packaging mechanism, which involves an elongated C-terminal tail of the VP, "pinning" the ssDNA genome to the capsid interior at the twofold symmetry axis. This mechanism fundamentally differs from the capsid-DNA interactions previously seen in parvoviruses. This study provides new insights on the mechanism behind ssDNA genome segmentation and on the plasticity of parvovirus biology.

Molecular biology and structure of a novel penaeid shrimp densovirus elucidate convergent parvoviral host capsid evolution

The giant tiger prawn (Penaeus monodon) is a decapod crustacean widely reared for human consumption. Currently, viruses of two distinct lineages of parvoviruses (PVs, family Parvoviridae; subfamily Hamaparvovirinae) infect penaeid shrimp. Here, a PV was isolated and cloned from Vietnamese P. monodon specimens, designated Penaeus monodon metallodensovirus (PmMDV). This is the first member of a third divergent lineage shown to infect penaeid decapods. PmMDV has a transcription strategy unique among invertebrate PVs, using extensive alternative splicing and incorporating transcription elements characteristic of vertebrate-infecting PVs. The PmMDV proteins have no significant sequence similarity with other PVs, except for an SF3 helicase domain in its nonstructural protein. Its capsid structure, determined by cryoelectron microscopy to 3-Å resolution, has a similar surface morphology to Penaeus stylirostris densovirus, despite the lack of significant capsid viral protein (VP) sequence similarity. Unlike other PVs, PmMDV folds its VP without incorporating a βA strand and displayed unique multimer interactions, including the incorporation of a Ca²⁺ cation, attaching the N termini under the icosahedral fivefold symmetry axis, and forming a basket-like pentamer helix bundle. While the PmMDV VP sequence lacks a canonical phospholipase A2 domain, the structure of an EDTA-treated capsid, determined to 2.8-Å resolution, suggests an alternative membrane-penetrating cation-dependent mechanism in its N-terminal region. PmMDV is an observed example of convergent evolution among invertebrate PVs with respect to host-driven capsid structure and unique as a PV showing a cation-sensitive/dependent basket structure for an alternative endosomal egress.

Characterization of the RNA Transcription Profile of Bombyx mori Bidensovirus

Bombyx mori bidensovirus (BmBDV) is a single-stranded DNA (ssDNA) virus from the genus Bidensovirus of the Bidnaviridae family, which, thus far, solely infects insects. It has a unique genome that contains bipartite DNA molecules (VD1 and VD2). In this study, we explored the detailed transcription mapping of the complete BmBDV genome (VD1 and VD2) by rapid amplification of cDNA ends (RACE), reverse transcription quantitative real-time PCR (RT-qPCR), and luciferase assays. For the first time, we report the transcription map of VD2. Our mapping of the transcriptional start sites reveals that the NS genes in VD1 have separate transcripts that are derived from overlapping promoters, P5 and P5.5. Thus, our study provides a strategy for alternative promoter usage in the expression of BmBDV genes.

Reprint of: Diversity of small, single-stranded DNA viruses of invertebrates and their chaotic evolutionary past

A wide spectrum of invertebrates is susceptible to various single-stranded DNA viruses. Their relative simplicity of replication and dependence on actively dividing cells makes them highly pathogenic for many invertebrates (Hexapoda, Decapoda, etc.). We present their taxonomical classification and describe the evolutionary relationships between various groups of invertebrate-infecting viruses, their high degree of recombination, and their relationship to viruses infecting mammals or other vertebrates. They share characteristics of the viruses within the various families, including structure of the virus particle, genome properties, and gene expression strategy.

Structural proteins of Helicoverpa armigera densovirus 2 enhance transcription of viral genes through transactivation

Herein, we report the identification of putative promoters for the non-structural proteins (NS) and capsid structural proteins (VP) of Helicoverpa armigera densovirus (HaDV2) as well as a potential mechanism for how these promoters might be regulated. For the first time, we report that VP is able to transactivate the VP promoter and, to a lesser degree, the NS promoter in densoviruses. In addition to this, another promoter-like sequence designated P2, when co-transfected with the VP gene, enhanced luciferase activity by approximately 35 times compared to a control. This suggests that there are two promoters for VP in HaDV2 and that the VP of parvoviruses might play a more important role in viral transcription than previously appreciated.

Diversity of small, single-stranded DNA viruses of invertebrates and their chaotic evolutionary past

A wide spectrum of invertebrates is susceptible to various single-stranded DNA viruses. Their relative simplicity of replication and dependence on actively dividing cells makes them highly pathogenic for many invertebrates (Hexapoda, Decapoda, etc.). We present their taxonomical classification and describe the evolutionary relationships between various groups of invertebrate-infecting viruses, their high degree of recombination, and their relationship to viruses infecting mammals or other vertebrates. They share characteristics of the viruses within the various families, including structure of the virus particle, genome properties, and gene expression strategy.

Effects of recombination on densovirus phylogeny

Densoviruses are a group of arthropod-infecting viruses with a small single-stranded linear DNA genome. These viruses constitute the subfamily Densovirinae of the family Parvoviridae. While recombination in between vertebrate-infecting parvoviruses has been investigated, to date, no systematic analysis of recombination has been carried out for densoviruses. The aim of the present work was to study possible recombination events in the evolutionary history of densoviruses and to assess possible effects of recombination on phylogenies inferred using amino acid sequences of nonstructural (NS) and capsid (viral protein, VP) proteins. For this purpose, the complete or nearly complete genome nucleotide sequences of 40 densoviruses from the GenBank database were used to construct a phylogenetic cladogram. The viruses under study clustered into five distinct groups corresponding to the five currently accepted genera. Recombination within each group was studied independently. The RDP4 software revealed three statistically highly credible recombination events, two of which involved viruses of the genus Ambidensovirus, and the other, viruses from the genus Iteradensovirus. These recombination events led to mismatches between phylogenetic trees constructed using comparison of amino acid sequences of proteins encoded by genome regions of recombinant and non-recombinant origin (regulatory NS1 and NS3 proteins and capsid VP protein).

Gene expression of five different iteradensoviruses: Bombyx mori densovirus, Casphalia extranea densovirus, Papilio polyxenes densovirus, Sibine fusca densovirus, and Danaus plexippus densovirus

Iteradensoviruses are 5-kb parvoviruses with typical J-shaped inverted terminal repeats of about 250 nucleotides and terminal hairpins of about 165 nucleotides. The single-stranded DNA genome contains several open reading frames, but their expression strategy is still unknown. Here the transcription maps and expression of the viruses in this genus were explored. As for brevidensoviruses, the two nonstructural (NS) genes were expressed by overlapping promoters with alternate transcription starts at both sides of the NS1 start codon.