Cross-Species Functionality of Pararetroviral Elements Driving Ribosome Shunting

Background: Cauliflower mosaic virus (CaMV) and Rice tungro bacilliform virus (RTBV) belong to distinct genera of pararetroviruses infecting dicot and monocot plants, respectively. In both viruses, polycistronic translation of pregenomic (pg) RNA is initiated by shunting ribosomes that bypass a large region of the pgRNA leader with several short (s) ORFs and a stable stem-loop structure. The shunt requires translation of a 5'-proximal sORF terminating near the stem. In CaMV, mutations knocking out this sORF nearly abolish shunting and virus viability. Methodology/Principal Findings: Here we show that two distant regions of the CaMV leader that form a minimal shunt configuration comprising the sORF, a bottom part of the stem, and a shunt landing sequence can be replaced by heterologous sequences that form a structurally similar configuration in RTBV without any dramatic effect on shunt-mediated translation and CaMV infectivity. The CaMV-RTBV chimeric leader sequence was largely stable over five viral passages in turnip plants: a few alterations that did eventually occur in the virus progenies are indicative of fine tuning of the chimeric sequence during adaptation to a new host. Conclusions/Significance: Our findings demonstrate cross-species functionality of pararetroviral cis-elements driving ribosome shunting and evolutionary conservation of the shunt mechanism. We are grateful to Matthias Muller and Sandra Pauli for technical assistance. This work was initiated at Friedrich Miescher Institute (Basel, Switzerland). We thank Prof. Thomas Boller for hosting the group at the Institute of Botany.