Selective encapsidation of CAT gene transcripts in TMV-infected transgenic tobacco inhibits CAT synthesis

Young tobacco seedlings (F1-progeny), transformed to express chloramphenicol acetyltransferase (CAT) mRNA with or without a 3'-proximal copy of the origin-of-assembly sequence (OAS) from tobacco mosaic virus (TMV) RNA (residues 5118-5550), were inoculated with TMV. After 21 days, virus symptoms were observed and systemic TMV infections were confirmed by Western blotting for viral coat protein and by electron microscopy of leaf saps. CAT activities were measured in extracts of leaf discs taken before, and 21 days after, virus inoculation. On average, the systemic leaves from TMV-infected CAT-transgenic plants containing the OAS exhibited 3.2-fold less CAT activity than the equivalent leaves from CAT-transgenic control plants lacking the OAS. Hence selective, OAS-dependent encapsidation of nuclear DNA transcripts into TMV-like (pseudovirus) particles can reduce expression of a particular mRNA, post-transcriptionally, in vivo. Furthermore, these data indicate that TMV self-assembly is not restricted to an exclusive subcellular compartment in vivo, and that formation of natural pseudovirions (A. Siegel, Virology 46, 50-59 (1971)) may shut off specific host RNA functions.