The Autographa californica Multiple Nucleopolyhedrovirus Gene Contains a -Acting Element That Is Essential for Nucleocapsid Assembly.

is a family of insect-specific viruses that have a circular double-stranded DNA genome packaged within a rod-shaped capsid. The mechanism of baculovirus nucleocapsid assembly remains unclear. Previous studies have shown that deletion of the gene of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) blocks viral nucleocapsid assembly. Interestingly, the -encoded protein Ac83 is not a component of the nucleocapsid, implying a particular role for in nucleocapsid assembly that may be independent of its protein product. To examine this possibility, Ac83 synthesis was disrupted by insertion of a chloramphenicol resistance gene into its coding sequence or by deleting its promoter and translation start codon. Both mutants produced progeny viruses normally, indicating that the Ac83 protein is not required for nucleocapsid assembly. Subsequently, complementation assays showed that the production of progeny viruses required the presence of in the AcMNPV genome instead of its presence in Therefore, we reasoned that is involved in nucleocapsid assembly via an internal -acting element, which we named the nucleocapsid assembly-essential element (NAE). The NAE was identified to lie within nucleotides 1651 to 1850 of and had 8 conserved A/T-rich regions. Sequences homologous to the NAE were found only in alphabaculoviruses and have a conserved positional relationship with another essential -acting element that was recently identified. The identification of the NAE may help to connect the data of viral -acting elements and related proteins in the baculovirus nucleocapsid assembly, which is important for elucidating DNA-protein interaction events during this process. Virus nucleocapsid assembly usually requires specific -acting elements in the viral genome for various processes, such as the selection of the viral genome from the cellular nucleic acids, the cleavage of concatemeric viral genome replication intermediates, and the encapsidation of the viral genome into procapsids. In linear DNA viruses, such elements generally locate at the ends of the viral genome; however, most of these elements remain unidentified in circular DNA viruses (including baculovirus) due to their circular genomic conformation. Here, we identified a nucleocapsid assembly-essential element in the AcMNPV (the archetype of baculovirus) genome. This finding provides an important reference for studies of nucleocapsid assembly-related elements in baculoviruses and other circular DNA viruses. Moreover, as most of the previous studies of baculovirus nucleocapsid assembly have been focused on viral proteins, our study provides a novel entry point to investigate this mechanism via -acting elements in the viral genome.