Ocs-elements are a group of related, bipartite promoter elements which have been exploited by two distinct groups of plant pathogens, Agrobacterium and certain viruses to express genes in plants. The genes for two Arabidopsis bZIP (basic region-leucine zipper) proteins that bind to ocs-elements have been isolated and characterized. The genes, called OBF4 and OBF5, were isolated by screening an Arabidopsis genomic library with degenerate oligonucleotides complementary to the DNA-binding domains of other plant ocs-element-binding proteins. The OBF4 and OBF5 proteins show 53% amino acid identity but low DNA homology. Southern blot analysis demonstrated that each of the OBF genes is a member of a small family. OBF4 is more similar to the tobacco TGA1a and Arabidopsis TGA1 proteins, while OBF5 is more similar to the maize OBF3.1, wheat HBP1b and Arabidopsis aHBP1b proteins. The DNA-binding properties of OBF4 and OBF5 were similar although OBF5 was able to bind simultaneously to both halves of the ocs-element more efficiently than OBF4. This difference in binding to the ocs-element between two closely related proteins from the same species is potentially significant since binding to both halves of the ocs-element is a pre-requisite for in vivo transcriptional activity.