LIGHT-MODULATION OF FERREDOXIN MESSENGER-RNA ABUNDANCE REQUIRES AN OPEN READING FRAME

Ferredoxin I (Fed-1) mRNA abundance is modulated by an internal light regulatory element that includes sequences both 5' and 3' of the trans lational initiation site. To test the hypothesis that the light respon se mediated by this element might be coupled to translation, we transf ormed tobacco plants with gene constructs blocked in translational ini tiation or elongation. Here, we report that such mutations abolish the light response in vivo. A nonsense mutation could be rescued by resto ring the open reading frame with a different sequence, even when the n ew codon caused an amino acid substitution. Our data establish that th e light response requires a translatable reading frame and thus provid e strong circumstantial evidence for post-transcriptional modulation o f Fed-1 mRNA levels. The Fed-1 system is presently the only higher pla nt example of a developmentally regulated change in mRNA abundance tha t requires translation of the affected mRNA.