FIG. 1.

Schematic representation of TBSV genome and various defective viral RNAs. The wt TBSV genome (T-100) is shown at the top as a thick horizontal line, with coding regions depicted as boxes which include the approximate molecular weights (in thousands) of the encoded proteins (6). The translation product p33 and its readthrough product p92 are indicated as arrows below the genome, and the approximate position of region 3.5 is shown as a small black box near the 3′ end of the genome. HS-175 is a mutant of the genome in which the p33 termination codon has been replaced by a tyrosine codon to allow the expression of p92 (arrow) but not p33 (22). Various smaller defective viral RNAs are depicted in which shaded boxes correspond to regions of the genome retained in these molecules whereas thin horizontal lines correspond to genomic segments which are absent. DI-82 and DI-83 both encode p33 (open box) and are identical, except that region 3.5 is deleted from DI-82 (28). DI-83ΔMfeI is structurally similar to DI-83 but encodes a C-terminally truncated form of p33 (open box), which contains a C-terminal extension of 18 aa from a different reading frame (adjoining black box). DI-72 and DI-73 are composed of four and three noncontiguous regions, respectively (i.e., regions I through IV and I through III/IV, respectively), encode no functional proteins, and are identical except that region 3.5 is absent in DI-72 (28).