5'-leader of a photosystem I gene in Nicotiana sylvestris, psaDb, contains a translational enhancer.

Messenger RNA primary structures responsible for translational efficiency of a photosystem I gene, psaDb, of Nicotiana sylvestris were studied using a transgenic tobacco system. The entire 5'-leader (23 base pairs) with the first four amino acid codons of the protein coding region was fused in frame with the beta-glucuronidase (GUS) gene under the control of the 35 S promoter of cauliflower mosaic virus (CaMV). This construct (CaMV::psaDb-GUS') was introduced into tobacco. GUS activity and GUS mRNA levels were determined for individual transformants, revealing that the insertion of the psaDb sequence greatly enhanced the GUS activity relative to GUS mRNA abundance. The GUS activity/GUS mRNA was 14 times higher in the CaMV::psaDb-GUS' transformants than in the control CaMV::GUS' transformants. The high GUS activity/GUS mRNA of the CaMV::psaDb-GUS' transformants was reduced 20-fold when 13 bases within the psaDb leader were altered. These 13 bases are common to the leaders of an Arabidopsis ferredoxin gene and the psaDb gene of N. sylvestris. Since GUS proteins encoded by these chimeric GUS genes have identical amino acid sequences, these results indicate that the 5'-leader of the psaDb mRNA contains a translational enhancer element.