A 42 bp promoter fragment of the gene for subunit III of photosystem I (psaF) is crucial for its activity.

The promoter of the gene for the subunit III of photosystem I reaction center (psaF) from spinach has been dissected and studied via promoter/GUS gene fusions in transgenic tobacco. It possesses an architecture that differs from any other spinach promoter of genes encoding proteins involved in photosynthesis studied to date. A 42 bp region located between -220 and -179 bp upstream of the transcription start site has been identified that is indispensable for expression and binds a trans-acting factor. Maximal light-response is obtained with a -220/+ 163 bp segment, whereas longer promoter sequences are significantly less effective, indicating the existence of upstream elements with silencer characteristics. F1 seedlings show different spatial expression patterns in darkness or light. Etiolated seedlings display high GUS activity in the upper hypocotyl, the hook region and the vascular tissue of the cotyledons, whereas in light-grown seedlings no activity was detected in the hypocotyl and almost all cells of the cotyledons express the GUS gene.