The effect of light on the expression of the Arabidopsis thaliana ferredoxin gene (fedA) was studied in mature tobacco plants. In light-treated leaves of tobacco plants transformed with a full-length ferredoxin gene, fedA-specific mRNA levels were more than twenty fold higher than in dark-treated controls. This indicates that all components for regulation of the Arabidopsis ferredoxin gene are present in tobacco. To identify light-regulatory elements in the fedA gene, we have tested a set of chimeric genes containing various parts of the fedA gene for light-dependent expression in mature tobacco plants. A fedA promoter-GUS fusion gene was not light-responsive, indicating that the 5'-upstream promoter region is not sufficient for light regulation. Fusion genes in which different transcribed regions of the fedA gene were expressed from the CaMV 35S promoter showed only limited light regulation, if any at all. This indicates that, like the fedA upstream region, the region downstream of the transcription start site is also not sufficient for full light regulation. The combined results suggest that for full light-regulated expression of the fedA gene, both the promoter region and sequences downstream of the transcription start site are required.