hycu-hr6, a large homologous region of Hyphantria cunea nucleopolyhedrovirus, is dispensable for virus replication in culture cells

In transient expression assays, hycu-hr6, the largest homologous region (hr) among the six hrs identified in the genome of Hyphantria cunea nucleopolyhedrovirus (HycuNPV) has been shown to stimulate the activity of an early promoter of the HycuNPV gp64 gene (hycu-gp64) which is located immediately downstream of hycu-hr6. In this study, we generated a recombinant HycuNPV defective in hycu-hr6 (vHycudeltahr6) and characterized its biological properties in permissive Splm cells. Analyses by Northern blotting and immunoblotting showed that the transcription of hycu-gp64 in vHycudeltahr6-infected Splm cells was significantly lower than that in HycuNPV-infected Splm cells until 12 h postinfection, and the observed difference in hycu-gp64 transcription was reflected in the production of Hycu-GP64 protein in the infected cells. In later stages of viral infection, however, no substantial difference was observed between vHycudeltahr6 and HycuNPV in both hycu-gp64 transcription and Hycu-GP64 protein production. Viral DNA replication also showed no substantial difference between vHycudeltahr6 and HycuNPV throughout the experiments. Furthermore, there were little, if any, differences in the ultimate yields of budded virions (BVs) and polyhedrin between vHycudeltahr6 and HycuNPV. These results demonstrate that vHycudeltahr6 is able to complete its infection cycle in Splm cells and yields BVs and polyhedrin at levels comparable to those of HycuNPV, indicating that hycu-hr6 is a dispensable viral element for HycuNPV replication in Splm cells.