Comparison of the genomes of two sympatric iridescent viruses (types 9 and 16)

Family Iridoviridae: poor viral relations no longer

Members of the family Iridoviridae infect a diverse array of invertebrate and cold-blooded vertebrate hosts and are currently viewed as emerging pathogens of fish and amphibians. Iridovirid replication is unique and involves both nuclear and cytoplasmic compartments, a circularly permuted, terminally redundant genome that, in the case of vertebrate iridoviruses, is also highly methylated, and the efficient shutoff of host macromolecular synthesis. Although initially neglected largely due to the perceived lack of health, environmental, and economic concerns, members of the genus Ranavirus, and the newly recognized genus Megalocytivirus, are rapidly attracting growing interest due to their involvement in amphibian population declines and their adverse impacts on aquaculture. Herein we describe the molecular and genetic basis of viral replication, pathogenesis, and immunity, and discuss viral ecology with reference to members from each of the invertebrate and vertebrate genera.

Genome of invertebrate iridescent virus type 3 (mosquito iridescent virus)

Iridoviruses (IVs) are classified into five genera: Iridovirus and Chloriridovirus, whose members infect invertebrates, and Ranavirus, Lymphocystivirus, and Megalocytivirus, whose members infect vertebrates. Until now, Chloriridovirus was the only IV genus for which a representative and complete genomic sequence was not available. Here, we report the genome sequence and comparative analysis of a field isolate of Invertebrate iridescent virus type 3 (IIV-3), also known as mosquito iridescent virus, currently the sole member of the genus Chloriridovirus. Approximately 20% of the 190-kbp IIV-3 genome was repetitive DNA, with DNA repeats localized in 15 apparently noncoding regions. Of the 126 predicted IIV-3 genes, 27 had homologues in all currently sequenced IVs, suggesting a genetic core for the family Iridoviridae. Fifty-two IIV-3 genes, including those encoding DNA topoisomerase II, NAD-dependent DNA ligase, SF1 helicase, IAP, and BRO protein, are present in IIV-6 (Chilo iridescent virus, prototype species of the genus Iridovirus) but not in vertebrate IVs, likely reflecting distinct evolutionary histories for vertebrate and invertebrate IVs and potentially indicative of genes that function in aspects of virus-invertebrate host interactions. Thirty-three IIV-3 genes lack homologues in other IVs. Most of these encode proteins of unknown function but also encode IIV3-053L, a protein with similarity to DNA-dependent RNA polymerase subunit 7; IIV3-044L, a putative serine/threonine protein kinase; and IIV3-080R, a protein with similarity to poxvirus MutT-like proteins. The absence of genes present in other IVs, including IIV-6; the lack of obvious colinearity with any sequenced IV; the low levels of amino acid identity of predicted proteins to IV homologues; and phylogenetic analyses of conserved proteins indicate that IIV-3 is distantly related to other IV genera.

Comparison of the major capsid protein genes, terminal redundancies, and DNA-DNA homologies of two New Zealand iridoviruses

Molecular comparisons were carried out on two iridoviruses isolated from endemic sympatric New Zealand pasture pests. These viruses, Costelytra zealandica iridescent virus (CzIV/IV16) and Wiseana iridescent virus (WIV/IV9), belong to the same virus genus but it is not known how related they are. The major capsid protein (MCP) gene from each virus was located, sequenced, and compared to the homologous gene from other iridoviruses. The MCP genes of WIV and CzIV were similar to each other (87.9% amino acid similarity) and to other iridovirus MCP genes. The MCP genes of both WIV and CzIV were most homologous to the MCP gene from Tipula iridescent virus (TIV/IV1), with amino acid similarities of 92.3 and 88.3% respectively. The genomes of WIV and CzIV were compared to other invertebrate iridoviruses using solution DNA-DNA hybridisations. Even after reducing the annealing stringency conditions hybridisation ratios never exceeded 10% indicating there is little sequence conservation between iridovirus genomes. Estimates of the size of terminal redundancies were also calculated for these viruses using pulsed-field agarose gel electrophoresis. These values ranged from 0 to 8%. These studies indicate that WIV and CzIV have distinct genomes and that the genus Iridovirus is comprised of a group of genetically diverse viruses.

RNA transcript mapping of the Wiseana iridescent virus genome

The replication of Wiseana iridescent virus (WIV) was studied in Lymantria dispar tissue culture cells. Using a combination of [35S]methionine pulse-labeling and Northern blotting with WIV DNA probes, a transcriptional map of the genome was constructed. WIV has a wide dispersal of immediate-early genes with seven different regions identified. WIV has been reported to have extensive repetitive DNA sequences but no early transcription was observed in these regions. Although fine-mapping is required, some early regions (Bam L and Eco O) have been identified which are transcriptionally active at 6- and 12-h but are shut down by 24 h. These regions could provide probes for early genes and the hypothesized switch from nuclear to cytoplasmic replication for iridoviruses.

A comparison of the structural polypeptides of three iridescent viruses (types 6, 9, and 16) and the mapping of the DNA region coding for their major capsid polypeptides

The iridoviruses from Wiseana cervinata (WIV, type 9), Costelytra zealandica (CzIV, type 16) and Chilo suppressalis (CIV, type 6) were compared by SDS-PAGE and Western protein blotting for antigenic determinants. The major capsid proteins were isolated and oligonucleotide probes were synthesized from the partial amino acid sequences. The DNA regions coding for the major capsid proteins of WIV (VP52), CzIV (VP53) and CIV (VP50) were located by hybridization of the oligonucleotide probes to blots of the viral DNA. The major capsid protein was used as the zero point for the proposed linearized maps of these viruses. Using antibody and 125I-labelling, several proteins were identified as being on the surface of the virion. It was also shown that CIV was not as antigenically distinct from these two viruses as previously reported.