Résumé
Cauliflower mosaic virus (CaMV) is a DNA-containing pararetrovirus
replicating by means of reverse transcription of a terminally redundant
pregenomic 35S RNA that is also used as a polycistronic mRNA. The
leader of 35S RNA is long, highly structured, and contains multiple
short ORFs (sORFs), which strongly interfere with the ribosome scanning
process. Translation of this RNA is initiated by a ribosome shunt
mechanism, in which ribosomes translate the most 5′-proximal short ORF
(sORF A), then skip a large region of the leader containing a putative
RNA encapsidation signal and reinitiate translation at the first long
viral ORF. Here, we demonstrate that the efficiency of the sORF
A-mediated ribosome shunt is an important determinant of viral
infectivity. Point mutations in sORF A, which reduced the basal level
of shunt-dependent expression and the degree of shunt enhancement by a
CaMV-encoded translation transactivator (TAV), consequently reduced
infectivity of the virus in turnip plants. First- or second-site
reversions appeared in the viral progeny. The second-site reversions
restored shuntdependent expression to an extent correlating with
their relative abundance in the progeny. Mutations that abolished both
the basal and TAV-activated components of shunting proved to be lethal.
Finally, by using an artificial stem structure that blocks scanning, we
obtained direct evidence that ribosome shunt operates during CaMV
infection.