LIGHT-REGULATED EXPRESSION OF THE ARABIDOPSIS-THALIANA FERREDOXIN-A GENE INVOLVES BOTH TRANSCRIPTIONAL AND POSTTRANSCRIPTIONAL PROCESSES

Ferredoxin is part of the photosynthetic apparatus of the chloroplast and is encoded in the nucleus. In Arabidopsis thaliana expression of t he ferredoxin A gene is influenced by both the presence of chloroplast s and light. Tobacco plants transformed with a ferredoxin promoter-GUS fusion gene showed a tissue-specific and light-dependent expression p attern. The effect of light on the expression of the fusion gene in tr ansgenic seedlings was only two- to fourfold, which is less pronounced than the 20-fold effect in Arabidopsis itself. Run-on transcription a ssays with nuclei isolated from Arabidopsis revealed a twofold modulat ion of transcriptional activity of the ferredoxin A gene under the inf luence of light. These results suggest the involvement of post-transcr iptional processes in light-regulated gene expression. A ferredoxin pr omoter deletion series ranging from -1205 to -43 was studied. All but the smallest deletion construct (at position -143 relative to the tran slation start site) showed comparable expression levels in mature leav es, 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, paravascula r tissue, epidermal and stomatal guard cells and trichomes. Transgenic seedlings treated with norflurazon, which blocks the development of g reen chloroplasts, showed a two- to fourfold reduction in GUS expressi on for all constructs. In Arabidopsis seedlings the effect of norflura zon on the expression of the ferredoxin A was eightfold. This again ca n be explained by the need for post-transcriptional processes of the r egulated gene expression of Arabidopsis ferrodoxin A.