Light‐regulated expression of the Arabidopsis thaliana ferredoxin A gene involves both transcriptional and post‐transcriptional processes

Ferredoxin is part of the photosynthetic apparatus of the chloroplast and is encoded in the nucleus. In Arabidopsis thaliana expression of the ferredoxin A gene is influenced by both the presence of chloroplasts and light. Tobacco plants transformed with a ferredoxin promoter–GUS fusion gene showed a tissue‐specific and light‐dependent expression pattern. The effect of light on the expression of the fusion gene in transgenic seedlings was only two‐ to fourfold, which is less pronounced than the 20‐fold effect in Arabidopsis itself. Run‐on transcription assays with nuclei isolated from Arabidopsis revealed a twofold modulation of transcriptional activity of the ferredoxin A gene under the influence of light. These results suggest the involvement of post‐transcriptional processes in light‐regulated gene expression. A ferredoxin promoter deletion series ranging from ‐1205 to ‐143 was studied. All but the smallest deletion construct (at position ‐143 relative to the translation start site) showed comparable expression levels in mature leaves, suggesting the presence of a positive regulating element between ‐269 and ‐143. The same pattern of tissue specificity was found in all promoter deletions studied. Expression of the fusion genes is high in all chloroplast‐containing cells: mesophyll, chlorenchyma, paravascular tissue, epidermal and stomatal guard cells and trichomes. Transgenic seedlings treated with norflurazon, which blocks the development of green chloroplasts, showed a two‐ to fourfold reduction in GUS expression for all constructs. In Arabidopsis seedlings the effect of norflurazon on the expression of the ferredoxin A was eightfold. This again can be explained by the need for post‐transcriptional processes of the regulated gene expression of Arabidopsis ferredoxin A.