EFFICIENT TRANSLATION OF DISTAL CISTRONS OF A POLYCISTRONIC MESSENGER-RNA OF A PLANT PARARETROVIRUS REQUIRES A COMPATIBLE INTERACTION BETWEEN THE MESSENGER-RNA 3'-END AND THE PROTEINACEOUS TRANSACTIVATOR

Caulimoviruses, a type of plant pararetrovirus, employ a highly unusua l mechanism to express the multiple cistrons of their pregenomic RNA. It involves translation of a polycistronic mRNA utilizing cis-acting v iral RNA sequences and a transacting virus-encoded protein (P6). In ad dition to its role in polycistronic translation, the translational tra ns-activator protein P6 also activates its own expression from a monoc istronic subgenomic RNA. Using Nicotiana edwardsonii cell suspension p rotoplasts, we analyzed the ability of P6 proteins from three differen t caulimoviruses to activate viral RNA-based reporter constructs. Cis- acting elements present in figwort mosaic caulimovirus (FMV) are funct ional not only in the presence of the cognate P6 activator protein, bu t 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 p olycistronic expression of FMV are replaced by their counterparts from PCISV, reporter gene expression is only observed in the presence of P CISV P6. Derepression of monocistronic reporter constructs tailed with FMV or CaMV 3' proximal sequences is less efficient in the presence o f PCISV P6 than with either FMV or CaMV P6, but more efficient when th e constructs contain a cognate PCISV 3' cis-element. Efficient express ion of polycistronic and monocistronic caulimovirus mRNAs in plant cel ls thus requires compatible interactions between P6, a translational t rans-activator, and its cognate cis-element at the 3' end of the mRNA. (C) 1996 Academic Press, Inc.