Introduction

The tobamovirus group of plant viruses has 12 recognized members (Francki et al., 1991) sharing the following characteristics: infective particles are very stable, rigid rods, of about 300 × 18 nm, with a sedimentation coefficient of 190 S; each particle is built of about 2000 protein subunits of a single molecular species (Mr 17–18 × 103), helically (pitch 2.3 nm) arranged together with a monopartite, single-stranded, genomic RNA (Mr 2 × 106, or ∼ 6.5 kb) of messenger polarity.

The genomic RNA (gRNA) encodes at least four proteins: gRNA is the mRNA for a 126 kD protein (126 K), whose open reading frame (ORF) initiates 60-71 nt from the capped 5′ end, and for a readthrough protein of 183 kD (183 K). Both the 126 K and the 183 K are required for viral RNA replication, and found in them are consensus sequences for methyltransferases and helicases in the 126 K and for RNA-polymerases, in the 54 kD (54 K) readthrough portion of the 183 K. A third ORF encodes a 30 kD protein (MP), required for the cell-to-cell movement of the virus in the plant; the fourth ORF encodes the coat protein (CP). To the 3′ side of the CP ORF, there is a non-coding region (3′ ncr) 179-414 nt long, that may adopt a tRNA-like configuration and can be amino-acylated, for most tobamoviruses, with histidine. Both MP, CP (and perhaps the 54 K readthrough part of the 183 K), are translated from 3′ -coterminal subgenomic RNAs produced during replication.