A Novel DNA-Binding Motif, Hallmark of a New Family of Plant Transcription Factors

A novel family of plant-specific transcription factors is described. They are structurally related to DBP1 (for DNA-binding protein phosphatase 1), a new transcription factor recently characterized in tobacco (Nicotiana tabacum), which exhibits both sequence-specific DNA-binding and protein phosphatase activity (Carrasco et al., 2003). The C-terminal part of DBP factors shows high sequence similarity to protein phosphatases of the 2C class (PP2C; Smith and Walker, 1996), and recombinant tobacco DBP1 is indeed an active protein phosphatase (Carrasco et al., 2003). Within the N-terminal region of these factors, with no significant homology to known protein sequences, we have identified and functionally characterized a novel and highly conserved motif involved in sequence-specific DNA binding. Protein phosphorylation/dephosphorylation enables cells to rapidly and reversibly modulate transcription factor function, and thereby gene expression, in response to signaling stimuli (Whitmarsh and Davis, 2000; Kobor and Greenblatt, 2002). This modulation is accomplished by the coordinated activity of protein kinases and protein phosphatases. Originally considered to act merely by reversing the effects of protein kinases, protein phosphatases are now recognized to fulfill essential regulatory functions in signaling pathways (Luan, 2003). In the last years, biochemical and genetic studies have identified PP2Cs as negative modulators of stress-responsive signaling pathways in animals, yeast, and also in plants (Maeda et al., 1994; Sheen, 1996; Gaits et al., 1997; Meskiene et al., 1998; Takekawa et al., 1998; Gosti et al., 1999; Meskiene et al., 2003). The identified DBP1 protein from tobacco plants was found to participate in the transcriptional regulation of the expression of the CEVI1 gene (Carrasco et al., 2003), a defense-related gene that is induced in susceptible plants as a consequence of viral infection (Mayda et al., 2000). Plant PP2Cs are characterized by the presence of specific, sequence-unrelated N-terminal extensions of undefined function (Rodríguez, 1998). Thus, specific functional features of DBP1, like its DNA-binding capacity, likely rest on this region, although sequence analysis did not result in the identification of any obvious, previously characterized DNA-binding motif. Because of the unique combination of protein phosphatase activity and sequence-specific DNA binding within the same protein molecule, the identification of tobacco DBP1 unveiled a new mechanism of transcriptional control remaining to be fully elucidated.