Abstract
Baculoviruses generally produce two progeny phenotypes—the budded virus (BV) and the occlusion-derived virus (ODV)—and the intricate mechanisms that regulate the temporal synthesis of the two phenotypes are critical for the virus replication cycle, which are far from being clearly understood. FP25K was reported to be responsible for the regulation of BV/ODV, and the mutations within result in a decrease of normal ODVs formation and an increase of BVs production. In this study, we demonstrated that the increase of BV titer in an fp25k knockout recombinant (fp25k-negative) was a result of higher infectivity of BVs rather than an increased production of BVs. The constitution of the major structural proteins and genome of parental and fp25k-negative BVs were analyzed. The results showed that the integrity of the majority of DNA packaged into the fp25k-negative BVs was intact; i.e., the genomic DNA of fp25k-negative BV had better transformation and transfection efficiency than that of the parental virus, indicating more intact genomes in the virions. Although the analysis of proteins associated with BVs revealed that more envelope protein GP64 were incorporated into the fp25k-negative BVs, subsequent experiments suggested that overexpression of GP64 did not improve the titer of BVs. Thus, we conclude that the main reason for higher infectivity of BVs is due to better genome integrity, which benefits from the deletion of fp25k resulting in increased stability of the genome and produce a higher proportion of infectious BVs. FP25K acts as a negative factor for the infectivity of BV.
Highlights
Baculoviruses are a diverse group of large double strain DNA viruses targeting insects, which contain four genera: Alphabaculovirus, Betabaculovirus, Gammabaculovirus, and Deltabaculovirus
The common characteristics of the few polyhedra (FP) phenomenon are a decrease in the number of OBs, an increase in the production of budded virus (BV), and reduced numbers of completely enveloped occlusion-derived virus (ODV) [16, 35]
There are at least two hypotheses to explain this increased BV production: (1) that the nucleocapsids destined to form ODVs participate in BV formation, or (2) fp25k-negative virus produced BVs with higher infectivity [16]
Summary
Baculoviruses are a diverse group of large double strain DNA viruses targeting insects, which contain four genera: Alphabaculovirus, Betabaculovirus, Gammabaculovirus, and Deltabaculovirus. Except for members of the genus Gammabaculovirus, two morphologically distinct virion phenotypes are produced in the biphasic life cycle of baculoviruses: the budded virus (BV) and the occlusion-derived virus (ODV) [1, 2]. ODV enters the epithelial cells of the insect midgut through direct membrane fusion and initiates primary infection, while BV is transmitted from cell to cell, and is responsible for secondary and systemic infection [3, 4]. Baculoviruses have been successfully developed as bioinsecticides or eukaryotic expression vectors/gene therapy vectors. Genetic modification was applied to improve baculovirus as a more efficient expression vector [5]
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