A mutant (m-29) of nuclear polyhedrosis virus (AcNPV) grew in and cells but did not form typical intranuclear occlusion bodies (OB); instead, small particles (95 to 180 nm diam.) were produced in copious amounts within nuclei. Ultrastructural studies showed that the particles did not occlude enveloped nucleocapsids and that they lacked a macromolecular paracrystalline lattice and a structure equivalent to the occlusion body envelope. The particles within nuclei stained in an immunofluorescence test with anti-polyhedrin antibody and when extracted from cells the major polypeptide of a particle preparation was indistinguishable from polyhedrin when examined on SDS-polyacrylamide gels and had an identical peptide pattern following proteolysis with V8 protease. Other elements believed to be implicated in OB morphogenesis such as a proliferation of intranuclear membranes, enveloped bundles of nucleocapsids, patches of fibrous material and fibrous sheets were present in normal amounts. No alteration in the synthesis or processing of polypeptides was seen in mutant-infected cells. Analysis of m-29 DNA with HI, RI and dIII restriction endonucleases revealed that the dIII restriction site at the F/V junction of viral DNA was absent in the mutant. No other modifications in the restriction patterns were detected. It is proposed that an alteration in the amino acid sequence of polyhedrin towards the — NH terminus of the polypeptide may account for the growth characteristics of the mutant.


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