The Autographa californica Multiple Nucleopolyhedrovirus ac83 Gene Contains a cis -Acting Element That Is Essential for Nucleocapsid Assembly

Abstract

Baculoviridae 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 ac83 gene of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) blocks viral nucleocapsid assembly. Interestingly, the ac83-encoded protein Ac83 is not a component of the nucleocapsid, implying a particular role for ac83 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 ac83 in the AcMNPV genome instead of its presence in trans. Therefore, we reasoned that ac83 is involved in nucleocapsid assembly via an internal cis-acting element, which we named the nucleocapsid assembly-essential element (NAE). The NAE was identified to lie within nucleotides 1651 to 1850 of ac83 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 cis-acting element that was recently identified. The identification of the NAE may help to connect the data of viral cis-acting elements and related proteins in the baculovirus nucleocapsid assembly, which is important for elucidating DNA-protein interaction events during this process.IMPORTANCE Virus nucleocapsid assembly usually requires specific cis-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 cis-acting elements in the viral genome.

Baculovirus Per Os Infectivity Factor Complex: Components and Assembly

“…2I). For pif8, the majority of the sequence was deleted, except for a fragment of 325 nt at the 5= end and an essential region for nucleocapsid assembly in the middle of the gene (41). To ensure that the remaining sequence does not encode amino acids, the transcriptional initiation motif TAAG of pif8 was mutated to TAAA, and a T at nt 58 of the ORF was deleted to generate a stop codon and a frameshift mutation (Fig.…”

Autographa californica Multiple Nucleopolyhedrovirusorf13is Required for Efficient Nuclear Egress of Nucleocapsids

“…Several cis-acting elements have been identified in AcMNPV. For example, Ac83 was required for per os infectivity factor (PIF) complex formation and was deemed a true PIF (35), while ac83 is involved in nucleocapsid envelopment via an internal cis-acting element (36).…”

Nucleocapsid Assembly of Baculoviruses

“…Moreover, VP1054 was able to interact with the capsid protein 38K that catalyzed the dephosphorylation of P6.9 in genome packaging [62]. Interestingly, a cis -acting element named NAE containing eight conserved A/T-rich regions was identified in the ac83 coding region and is essential to DNA packaging [2]. Homologs of NAE are detected only in alphabaculoviruses.…”

“…Baculoviridae consists of a large family of circular dsDNA viruses with DNA molecules sized from 80 to 180 kbp that are encapsulated in enveloped rod-like capsids [1,2]. Baculoviruses are insect-specific pathogens that have been extensively applied for insect control [3].…”