Genetic engineering of a Lymantria dispar nuclear polyhedrosis virus for expression of foreign genes

A bacterial lacZ gene was inserted into an isolate of the Lymantria dispar nuclear polyhedrosis virus (LdMNPV). The transfer vector was constructed by site-directed mutagenesis of the translation start site of the LdMNPV polyhedrin gene, within the BglII E fragment of the viral genome. A multiple cloning sequence was inserted at this start site and used for the insertion of the lacZ gene into the transfer plasmid. Liposome transfection was used to cotransfect L. dispar tissue culture cells with viral DNA and the transfer plasmid. Recombinant LdMNPV isolates were purified by isolation of plaques producing beta-galactosidase but not polyhedra. Restriction enzyme fragment profiles were used to determine the site of the lacZ gene insertion, and DNA sequencing of the 5' and 3' ends of the lacZ gene insert and the adjoining polyhedrin promoter and coding regions was performed to identify its precise location. Expression of the lacZ gene was examined by studying virus-induced protein using [35S]methionine pulse-labelling, SDS-PAGE fractionation and autoradiography. Expression of beta-galactosidase was examined in tissue culture cells using colorimetric assays. The maximum rate of beta-galactosidase production was approximately 50 international units (IU)/10(6) tissue culture cells/day between 3 and 4 days post-infection (p.i), and the peak total expression was 158 IU/10(6) cells 5 days p.i. beta-Galactosidase activity was first detected 48 h p.i. in haemolymph samples from fourth instar L. dispar larvae injected with 10(6) p.f.u. of virus. The peak beta-galactosidase activity in larval haemolymph samples was 1931 IU/ml of haemolymph at 11 days p.i., just prior to death.