Abstract
The chloroplast atpB and atpE genes code for subunits beta and epsilon, respectively, of the chloroplast ATP synthase. The atpB stop codon (UGA) overlaps one nucleotide with the atpE start codon (AUG) in most flowering plants. The two genes are co-transcribed as a dicistronic atpB-atpE mRNA. Therefore, it was long thought that translation of the downstream atpE cistron depends on that of the upstream atpB cistron (i.e. translational coupling). Using two different fluorescent proteins, we devised an in vitro assay to measure differentially translation of the atpB and atpE cistrons. We first showed that both atpB and atpE cistrons are translated from the tobacco dicistronic mRNA by using our efficient in vitro translation system from tobacco chloroplasts. When the atpB 5-UTR was replaced with a lower efficient 5-UTR, atpE translation was higher than atpB translation. This result strongly suggests that atpE translation is independent of atpB translation (no translational coupling).
