Light Regulation of Fed-1 mRNA Requires an Element in the 5′ Untranslated Region and Correlates with Differential Polyribosome Association

Light regulation of Fed-1 mRNA abundance in the leaves of green plants is primarily a post-transcriptional process. Previously, we have shown that the Fed-1 mRNA light response requires an open reading frame, indicating that the light regulation of the mRNA depends on its concurrent translation. We now show that light-induced increases in Fed-1 mRNA abundance are associated with increases in polyribosome association that require both a functional AUG and a normal Fed-1 translational start context. We also present evidence that light regulation of Fed-1 mRNA levels requires more than efficient translation per se. Substitution of the efficiently translated tobacco mosaic virus Ω 5′ untranslated region resulted in a loss of Fed-1 light regulation. In addition, we identified a CATT repeat element located near the 5′ terminus of the Fed-1 5′ untranslated region that is essential for light regulation. We introduced two different mutations in the CATT repeat element, but only one of these substitutions blocked the normal light effect on polyribosome association, whereas both altered dark-induced Fed-1 mRNA disappearance. The element may thus be important for Fed-1 mRNA stability rather than polyribosome loading. We propose a model in which Fed-1 mRNA is relatively stable when it is associated with polyribosomes in illuminated plants but in darkness is not polyribosome associated and is thus rapidly degraded by a process involving the CATT repeat element.