Sequences involved in the control of translation in potato virus S.

Specific cis-acting sequences within the carlavirus potato virus S (PVS) genomic RNA molecule appear to control gene expression at the translational level. Two sequences have been investigated, the untranslated sequence upstream from the initiation codon of the viral coat protein gene, designated VTE and the 5' untranslated leader sequence from the genomic RNA molecule (PVS 5'). In vitro and in vivo, either of these sequences enhance the translation of a downstream open reading frame when provided as the untranslated leader in a transcript molecule. Translational enhancement was also detected at the transgenic plant level. Both PVS sequences were deleted in an attempt to identify the core regulatory element responsible for this translational enhancement phenomenon. Results indicate that in vitro and in vivo, the functional motif is contained within the 5' promixal portion of both sequences. When the sequences of these important regions were compared, a homologous block of nucleotides was identified, a block which is also highly conserved within the 5' untranslated leader of another carlavirus, blueberry scorch virus (BBScV). In addition to the function of translational enhancement, the VTE sequence, and a parallel sequence from another carlavirus, Helenium virus S (HelVS), direct internal ribosome entry and initiation of translation in vivo in a prokaryotic system and in vitro in a eukaryotic system, respectively. Results have also indicated that the mechanism of expression associated with the VTE and PVS 5' leader may be at least partially cap-independent in nature. This may represent an adaption by the virus to achieve a maximum rate of translation and multiplication when cellular translational machinery is depleted or altered upon viral infection. Preliminary investigation has also assigned additional functions to both sequences. The VTE sequence may contain the recognition site for the viral polymerase in the production of the PVS 1.3 kb subgenomic RNA molecule. The PVS 5' leader may contain the specific sequence or structure that is recognised by the viral coat protein in the initiation of genomic RNA encapsidation.