A Myosin-like Dimerization Helix and An Extra-large Homeodomain Are Essential Elements of the Tripartite Dna-binding Structure of Lfb1

The transcription activator LFB1 is a major determinant of hepatocyte-specific expression of many genes. To study the mechanisms underlying LFB1 transcriptional selectivity, we have initiated its biochemical characterization. By in vitro complementation assays we have defined two distinct regions required for high levels of transcription, which resemble previously described activation domains. In contrast, the region of LFB1 necessary for DNA binding displays several novel features. The DNA binding domain is tripartite, including a homeodomain of unusual length (81 amino acids) and an N-terminal helix similar to part of myosin. This helical region mediates dimerization, which is shown to be essential for DNA binding.