No abstract

Nucleopolyhedroviruses (NPVs), family Baculoviridae, are insect-specific viruses with the potential to control insect pests in agriculture and forestry. NPVs are occluded in polyhedral occlusion bodies. Polyhedra protect virions from inactivation in the environment as well as assisting virions in horizontal transmission in the insect population. The process of virion occlusion in the polyhedra is undefined and the genes that regulate the virion occlusion process have not been well investigated yet. An Autographa californica multiple nucleopolyhedrovirus (AcMNPV) mutant (AcDef) that has a 2136 bp DNA deletion, including p26, p10 and p74 genes, has been isolated. No virions were detected in the polyhedra of AcDef. Restoration of all the missing sequences into AcDef led to proper virion occlusion. Individual gene deletion of either p10 or p26 could not abolish virion occlusion in the polyhedra of AcMNPV, but p10 deletion reduced virion occlusion efficiency more than threefold compared with the wild-type AcMNPV. Previous studies by other research groups on deletion of AcMNPV gene p74 suggested that p74 is a per os infectivity factor, and deletion of the p74 gene did not eliminate virion occlusion. Collectively, the three genes (p26, p10 and p74) may act in concert to regulate the virion occlusion process. Therefore, p26, p10 and p74 are all required for proper virion occlusion in the polyhedra of AcMNPV.

The major capsid protein (mcp) gene of Spodoptera exigua ascovirus 5a (SeAV-5a) was confirmed by aphidicolin viral DNA replication inhibition analysis to be a late gene. The 5' and 3' ends of mcp gene transcripts have been mapped. Primer extension analyses indicated that transcription of the mcp gene initiates from a cytosine 25 nucleotides (nt) upstream of the translation start codon. Two independent approaches by 3' rapid amplification of cDNA ends (3' RACE) and oligo (dT) cellulose binding assay suggested that SeAV-5a mcp mRNA is polyadenylated. Analyses by 3' RACE also revealed that mcp transcripts terminate at a U, either at 26 or 38 nt downstream of the translation stop codon. The putative 5' transcription control region of the SeAV-5a mcp gene shares similarities with other ascoviruses and Chilo iridescent virus (CIV), containing a conserved TATA-box-like motif (TAATTAAA) and an ATTTGATCTT motif upstream of it. The 3' downstream regions of the mcp gene of all the ascoviruses examined and CIV can form a stem-loop structure, and the ends of the mcp gene transcripts of SeAV-5a are within the predicted stem-loop region. This suggests that the stem-loop structure of the mcp gene might be involved in transcription termination.