Expression of a chimeric uidA gene indicates that polycistronic mRNAs are efficiently translated in tobacco plastids

Polycistronic mRNAs are the predominant form of plastid primary transcripts. To determine if there is internal initiation of translation of promoter-distal open reading frames, a promoterless uidA reporter gene was integrated into the tobacco plastid genome downstream of the rbcL gene. Monocistronic uidA mRNA does not accumulate from the promoterless uidA construct. However, due to inefficient rbcL transcription termination, a polycistronic transcription unit is created that contains the uidA gene as the second cistron. Numerous stop codons in all three reading frames between the rbcL and uidA coding regions ensure that translation of uidA initiates only from the correct start codon. The encoded reporter gene product, beta-glucuronidase (GUS) accumulates to high levels in the transplastomic plants indicating that promoter-distal cistrons can be efficiently translated in plastids.