A virus mutant with an insertion of a copia-like transposable element

Abstract

The important role insect viruses have in the biological control of pests has inspired a considerable effort towards understanding their molecular biology. The nuclear polyhedrosis viruses (NPVs) are particularly interesting and accessible owing to their unique nuclear replication cycle and large (∼100 kilobase, kb) double-stranded, circular DNA genome. Serial passage of NPVs in cell culture results in a gradual reduction in the ability of the virus to carry out the biphasic growth cycle of producing extracellular virus particles (non-occluded virus, NOV), followed by the occlusion of virus particles in proteinaceous polyhedra within the nucleus. Instead, NOVs become the predominant replication product of serially passaged NPVs, with occlusion being a rarely observed, and probably aberrant, phenomenon^1–5. Plaque purification of Autographa californica (the alfalfa looper, Lepidoptera; Noctuidae) NPV (AcNPV) after 25 passages in cell cultures of Trichoplusia ni (the cabbage looper, Lepidoptera; Noctuidae) resulted in the isolation of few-polyhedra (FP) mutants. One of these plaque isolates, FP-D, exhibited a restriction endonuclease fragment pattern indicative of an insertion, probably of non-viral DNA, at 86.4–86.6 on the physical map. Of additional interest was the observation of restriction fragments in sub-molar quantities⁶. We have now characterized this mutant; it contains an insertion of a copia-like transposable element, designated TED, derived from the host T. ni cells.