Octopine synthase (ocs) elements are a group of promoter elements that have been exploited by plant pathogens to express genes in plants. ocs elements are components of the promoters of certain plant glutathione S-transferase genes and may function as oxidative stress response elements. Genes for ocs element binding factors (OBFs), which belong to a specific class of highly conserved, plant basic domain-leucine zipper transcription factors, have been isolated and include the Arabidopsis OBF4 and OBF5 genes. To characterize proteins that modulate the activity of the OBF proteins, we screened an Arabidopsis cDNA library with the labeled OBF4 protein and isolated OBP1 (for OBF binding protein). OBP1 contains a 51-amino acid domain that is highly conserved with two plant DNA binding proteins, which we refer to as the MOA domain. OBP1 is also a DNA binding protein and binds to the cauliflower mosaic virus 35S promoter at a site distinct from the ocs element in the 35S promoter. OBP1 specifically increased the binding of the OBF proteins to ocs element sequences, raising the possibility that interactions between these proteins are important for the activity of the 35S promoter.