Gene VI of Figwort Mosaic Virus Activates Expression of Internal Cistrons of the Full-length Polycistronic RNA Transcript

Abstract

Figwort mosaic virus (FMV) is a caulimovirus with a circular double-stranded DNA genome of about 8 kilobase pairs (kbp) (Richins et al., 1987). Like CaMV, the genome is expressed as two sets of genes (Gowda et al., 1989). The first set consists of a group of six genes (VII and I through V) closely spaced on a major polycistronic transcript which spans the entire viral genome. The second set consists of gene VI which has its own promoter and is transcribed as a separate monocistronic transcript (Fig. 1). This suggests that gene VI may have some early function in the replication cycle. The product of this gene of FMV accumulates in viral inclusion bodies, similar to those of cauliflower mosaic virus (CaMV), the type member of caulimovirus group. Although the role of gene VI of CaMV in disease induction and the host range determination is well documented (Daubert et al., 1984; Schoelz et al., 1986)), its molecular function has only recently been studied. As presented here and by Hohn et al., for CaMV in this volume, as well as a previous report (Hohn et al., 1989), gene VI appears to activate the expression of internal genes of the polycistronic transcript. This role has been demonstrated by the electroporation of viral genome-reporter gene constructs into plant protoplasts. For example, a plasmid containing the promoter of the major RNA transcript, the long intergenic region and gene VII (Fig. 2) followed by the chloramphenicol acetyltransferase (CAT) gene as a downstream cistron, shows little CAT activity following electroporation into plant protoplasts unless a separate plasmid expressing gene VI of FMV is included in the electroporation mixture.