Fig. 4.

Ribosome shunting, reinitiation and leaky scanning in members of the family Caulimoviridae. (a) Translation of ORF I of the pgRNA of rice tungro bacilliform tungrovirus is by ribosome shunting. Here, 40S complexes released after translation of the short 5′-most ORF (yellow) are able to shunt past a stable stem–loop in the 5′UTR (red arrow) and continue scanning the mRNA. Reinitiation subsequently takes place at the start codons of either ORF I (a non-AUG codon, AUU), ORF II (weak context AUG) or ORF III (strong context AUG). ORF IV is expressed from a spliced mRNA. (b) In cauliflower mosaic caulimovirus, a similar shunting mechanism is used to access the 5′-most coding ORF, VII. However, downstream ORFs I–V are translated via reinitiation events that are stimulated by a viral reinitiation factor, transactivator viroplasmin (TAV), expressed from an sgRNA (see text).