Calmodulin isoforms differentially enhance the binding of cauliflower nuclear proteins and recombinant TGA3 to a region derived from the Arabidopsis Cam-3 promoter

Many stimuli increase cytoplasmic Ca²⁺ concentrations as an early signal transduction event and alter the patterns of nuclear gene transcription, but the mechanisms by which Ca²⁺ signals are transduced to the nucleus are not known. This article shows that at least four DNA binding proteins from cauliflower nuclear extracts are also calmodulin (CaM) binding proteins. CaM enhances the binding of these proteins to a C/G-box sequence element in the Arabidopsis Cam-3 promoter. Binding to the C/G-box is enhanced preferentially by the CaM isoform encoded by Cam-3. However, it is not clear whether the effect is mediated directly by CaM or indirectly through the activity of a CaM-regulated protein phosphatase. CaM also binds recombinant TGA3 and enhances its binding to the same Cam-3 promoter element. These results are consistent with the idea that a Ca²⁺-mediated signaling pathway eliciting some changes in gene expression may consist of CaM, or a structurally related Ca²⁺ binding protein, and transcription factors.