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.