Replication and pathogenesis of an iridescent virus in the cotton boll weevil
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The boll weevil is a devastating pest of cotton and with increasing problems relating to chemical control, it is clear that biological approaches must be developed. Chilo iridescent virus (CIV) is the only virus known to infect the boll weevil, and research in our laboratory has shown that CIV induces up to 70% mortality and deformity in infected insects. The objectives of the present study were fourfold. The first objective was to demonstrate a complete replication cycle of CIV in the boll weevil and develop an infectivity assay. The second objective was to study inhibition of host protein synthesis and induction of apoptosis caused by CIV virion extracts. The third objective was to characterize a putative CIV homo log of the vaccinia virus BIR gene. The final objective was to establish protein kinase activity with virion extracts. Dot blot analysis of infected boll weevils provided strong evidence of viral DNA replication. Election microscopy established high levels of complete virus particles in infected cells. An infectivity assay (using viral DNA replication as indicator) was developed, and production of infectious progeny virus was demonstrated. Extracts prepared from CIV severely reduced protein synthesis in treated boll weevil and budworm cells as detected by SDS-PAGE analysis of labeled amino acid uptake into cellular proteins. The virion extracts also induced apoptosis in treated cells, which exhibited typical morphology of apoptosis as well as DNA fragmentation. Analysis of CIV DNA revealed the presence of an open reading frame with high similarity to the vaccinia virus BIR protein kinase gene. The BIR gene has been implicated in the shutdown of host macromolecular synthesis in cells infected with vaccinia virus possibly via phosphorylation of ribosomal proteins. Analysis of the CIV sequence revealed the two regions characteristic of protein kinases. Finally, in vitro assays established protein kinase activity in virion extracts. This activity was correlated with two polypeptides by FPLC. Our data suggest that CIV is a promising source of genes for the genetic engineering of boll weevil-resistant cotton plants.