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Engineered derivatives of Autographa californica multiple nucleocapsid nuclear polyhedrosis virus (AcMNPV) possessing a unique restriction site provide a source of viral DNA that can be linearized by digestion with a specific endonuclease. Circular or linearized DNA from two such viruses were compared in terms of their infectivity and recombinogenic activities. The linear forms were 15- to 150-fold less infectious than the corresponding circular forms, when transfected into Spodoptera frugiperda cells using the calcium phosphate method. Linear viral DNA was, however, proficient at recombination on co-transfection with an appropriate transfer vector. Up to 30% of the progeny viruses were recombinant, a 10-fold higher fraction of recombinants than was obtained from co-transfections with circular AcMNPV DNA. The isolation of a recombinant baculovirus expression vector from any of the AcMNPV transfer vectors currently in use can thus be facilitated by linearization of the viral DNA at the appropriate location.

A functional chitinase gene (chiA) has been identified in the genome of the Autographa californica nuclear polyhedrosis virus (AcMNPV). It is expressed in the late phase of virus replication in insect cells. High levels of both endo- and exochitinase activity were detected by 12 hr p.i. and remained stable throughout infection. An AcMNPV chiA protein-specific antibody was prepared using recombinant material prepared in bacteria. This was used to demonstrate that a product of approximately 58 kDa was synthesised in virus-infected cells. Immunofluorescence analysis of virus-infected cells showed that most chitinase was located in the cytoplasm. Primer extension analysis of mRNA from AcMNPV-infected cells confirmed that transcription initiated from a baculovirus late start site (TAAG), 14 nucleotides upstream from the putative translation initiation codon. The predicted protein sequence of the AcMNPV chiA shares extensive sequence similarity with chitinases from bacteria and, in particular, the Serratia marcescens chitinase A (60.5% identical residues). Phylogenetic analyses indicate that AcMNPV, or an ancestral baculovirus, acquired the chitinase gene from a bacterium via horizontal gene transfer.

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