Light-responsive elements of the tobacco PSI-D gene are located both upstream and within the transcribed region.

psaDb is a nuclear gene encoding the ferredoxin-binding subunit of photosystem I in Nicotiana sylvestris. The organization of the light-responsive cis-elements of psaDb was studied using transgenic tobacco plants. Three types of psaDb chimeric constructs were created: (1) a 5' upstream fragment of psaDb transcriptionally fused with the beta-glucuronidase (GUS) gene, and a series of its 5' deletion derivatives, (2) the transcribed region of psaDb driven by the cauliflower mosaic virus (CaMV) 35S promoter, and (3) the 5' terminal 35 bases (the entire leader, +1 to +23, and the initiation codon context, +24 to +35) of the psaDb mRNA translationally fused with a GUS reporter gene under the operation of the CaMV 35S promoter. Light-responsiveness of these fusions in transgenic plants was examined by GUS assay and primer extension analysis. The results indicate that the light-responsive elements (LRE) of psaDb are located both upstream (-170 to +24) and within (+1 to +861) the transcribed region. The internal LRE is utilized in etiolated seedings but not in green leaves. The leader and initiation codon context construct (+1 to +35) did not show any light-response under the conditions tested. Therefore, it is likely that a combination of the upstream and internal LREs generates the complex light-responsive and tissue-specific regulation of this gene. This study also revealed that psaDb has adjacent activator (-267 to -254) and repressor (-253 to -234) regions for basal transcriptional activity; the former contains the ACGT binding motif recognized by many plant bZIP proteins, and the latter has the R3 decamer motif found in several photosystem I-related genes.