Polycistronic mRNAs containing an upstream beta-glucuronidase (GUS) and a downstream chloramphenicol acetyltransferase (CAT) reporter open reading frame (ORF) were expressed in transfected plant protoplasts. CAT expression could be strongly induced by coexpression of the cauliflower mosaic virus encoded translation transactivator. Transactivation was abolished when an upstream ORF overlapped the CAT ORF for a long distance. No specific sequence elements were required for transactivation but the presence of a short ORF upstream of the GUS ORF strongly enhanced the process. The inhibitory effect of additional presumed stem structures inserted into various regions of the reporter mRNAs indicates that both ORFs are translated by ribosomes that associate with the RNA at the 5' end and reach the ORFs by a linear migration mechanism.