Efficient translation of distal cistrons of a polycistronic mRNA of a plant pararetrovirus requires a compatible interaction between the mRNA 3' end and the proteinaceous trans-activator

Caulimoviruses, a type of plant pararetrovirus, employ a highly unusual mechanism to express the multiple cistrons of their pregenomic RNA. It involves translation of a polycistronic mRNA utilizing cis-acting viral RNA sequences and a transacting virus-encoded protein (P6). In addition to its role in polycistronic translation, the translational trans-activator protein P6 also activates its own expression from a monocistronic subgenomic RNA. Using Nicotiana edwardsonii cell suspension protoplasts, we analyzed the ability of P6 proteins from three different caulimoviruses to activate viral RNA-based reporter constructs. Cis-acting elements present in figwort mosaic caulimovirus (FMV) are functional not only in the presence of the cognate P6 activator protein, but also in the presence oi the heterologous activators from cauliflower mosaic caulimovirus (CaMV) and peanut chlorotic streak caulimovirus (PCISV). However, when 3' cis-acting elements essential for efficient polycistronic expression of FMV are replaced by their counterparts from PCISV, reporter gene expression is only observed in the presence of PCISV P6. Derepression of monocistronic reporter constructs tailed with FMV or CaMV 3' proximal sequences is less efficient in the presence of PCISV P6 than with either FMV or CaMV P6, but more efficient when the constructs contain a cognate PCISV 3' cis-element. Efficient expression of polycistronic and monocistronic caulimovirus mRNAs in plant cells thus requires compatible interactions between P6, a translational trans-activator, and its cognate cis-element at the 3' end of the mRNA. (C) 1996 Academic Press, Inc.