Baculovirus infection of Spodoptera exigua larvae: lacZ expression driven by promoters of early genes pe38 and me53 in larval tissue.

To follow the progression of infection of Autographa californica multicapsid nuclear polyhedrosis virus (AcMNPV) within tissues of its larval host, we have constructed AcMNPV recombinants carrying lacZ reporter genes under the control of the early virus promoters pe38 and me53, in addition to the authentic genes. The early promoter-lacZ gene cassettes were located upstream of the very late polyhedrin gene. In infected insect cell lines, pe38 transcription is initiated at an early promoter, while me53 transcripts start from both early and late sites. Transcriptional mapping of the duplicated me53 and pe38 promoters driving lacZ expression showed that they initiated at the same start sites as in the authentic genes. Expression of lacZ by these recombinants was compared to a recombinant driving beta-glucuronidase expression from the very late p10 promoter and lacZ expression from the constitutive heat shock protein 70 promoter of Drosophila melanogaster. After infection of Spodoptera exigua larvae with the different recombinants, we followed reporter gene expression and polyhedron formation in different tissues using immunohistochemistry and electron microscopy. LacZ expression, indicative of early viral transcriptional activity, was detected in nearly all larval tissues during the course of infection. In most tissues these early events were followed by pathophysiological changes associated with late and very late gene expression. However, p10 transcription and polyhedron formation were not observed in midgut goblet cells, Malpighian tubules and salivary glands. These results suggest that expression of early virus genes, such as me53 and pe38, is not restricted to larval tissues that are permissive for AcMNPV replication.