Induction of apoptosis by iridovirus virion protein extract.

Chilo iridescent virus (CIV; IIV-6) is the type member of the genus Iridovirus (family Iridoviridae, large icosahedral cytoplasmic DNA viruses). CIV induces death and deformity in the cotton boll weevil, Anthonomus grandis, replicates productively in larvae of the cotton boll weevil, and significantly reduces laboratory populations of the cotton aphid, Aphis gossypii. CIV virion protein extract (CVPE) shuts down host protein synthesis in several insect cell lines and induces mortality in neonate boll weevil larvae. We report here that CVPE induces apoptosis in spruce budworm and boll weevil cell lines, as detected by blebbing, DNA fragmentation, and TUNEL assay. Tissue culture toxicity dose assays (TCTD(50)) showed that spruce budworm cells were eight times more sensitive to CVPE than boll weevil cells. Pancaspase inhibitor suppressed apoptosis but had marginal effect on inhibition of host protein synthesis. Moreover, the CVPE dose for apoptosis was 1000-fold lower than the dose for shutdown of host synthesis. We also detected protein kinase activity in CVPE. Heating CVPE at 60 degrees C for 30 min destroyed all three activities. Our results suggest that one or more polypeptides in CIV induce apoptosis. This is the first study demonstrating apoptosis induction by a member of the genus Iridovirus and by virion extracts of a member of the family Iridoviridae.

Transcriptional mapping in Chilo iridescent virus infections.

Chilo iridescent virus (CIV) belongs to the family Iridoviridae, which are icosahedral cytoplasmic DNA viruses with large, linear, and circularly permuted genomes. Previous studies on infected-cell-specific polypeptides suggested temporal regulation of CIV gene expression. Recently, we demonstrated three temporal classes at the transcriptional level, in CIV infections of a spruce budworm cell line. We also demonstrated a transcriptional cascade with positive and negative control. In this paper, we assign all detectable viral transcripts into respective temporal classes and map them using restriction fragments from a genomic library. More than 90 percent of the genome is transcriptionally active with at least four major clusters of immediate-early transcription and at least three delayed-early clusters. Late transcripts were observed throughout the genome. There was at least one exclusive region in the genome for each of the three temporal classes. We correlated transcribed regions with ORFs on the CIV genome and showed that known ORFs in the exclusive regions are generally consistent with phase-specific requirements of large DNA viruses. Our data also suggest the presence of 5' or 3' coterminal transcripts. This is the first complete transcription map for a member of the genus Iridovirus.

Transcription and temporal cascade in Chilo iridescent virus infected cells.

Chilo iridescent virus (CIV) is the type species for genus Iridovirus, and belongs to the family Iridoviridae. Members of this family are large, isometric, cytoplasmic DNA viruses. Our laboratory has established that CIV replicates productively in the cotton boll weevil, Anthonomus grandis. Given the economic importance of this host and the dearth of knowledge on this virus, we have initiated host-virus interaction and molecular studies on CIV. This report focuses on regulation of transcription in CIV infections. We carried out northern analyses on total cellular RNA from infections of IPRI-CF-124T cells, using a complete genomic library of CIV and several putative gene-specific probes. Our data show a temporal cascade based on analysis of 137 detectable transcripts comprising 38 immediate-early (IE), 34 delayed-early (DE), and 65 late (L) transcripts. Analysis with gene-specific probes supported the cascade pattern. Both helicase and RNA polymerase were immediate-early; major capsid protein was late. The CIV gene expression cascade appears to operate primarily at the transcriptional level. Temporal classes observed are consistent with earlier studies at the polypeptide level and with transcriptional patterns in frog virus 3, genus Ranavirus in the Iridoviridae. Our results provide an important basis for understanding mechanisms driving the CIV temporal cascade.