The function of mammalian transcription factors of the leucine zipper class was investigated in leaf-derived protoplasts of tobacco. In transient expression experiments, Fos and Jun strongly activated chimeric promoters composed of the TATA box region of the cauliflower mosaic virus 35S transcript preceded by one to five copies of an AP1/GCN4 binding site. Fos and Jun also stimulated a wheat high molecular weight glutenin promoter in which similar binding sites are located more than 500 base pairs from its transcription start site. Both the DNA binding and the transcription activation domains of these proteins were required for proper promoter stimulation by Fos and Jun. Each factor alone was partially active, suggesting that at least the Fos protein can associate with an endogenous plant transcription factor. These observations support the hypothesis that sequences related to AP1/GCN4 binding sites could be cis-acting modules involved in the transcriptional regulation of plant genes.