Calmodulins and related potential calcium sensors of Arabidopsis

•   Calcium (Ca²⁺ ) signaling is thought to orchestrate responses to cellular stimuli. The efficacy of Ca²⁺ signaling requires mediation by Ca²⁺ -binding proteins. •   The determination of the Arabidopsis genome sequence enables the identification of genes encoding potential Ca²⁺ sensors. •   Six Arabidopsis loci are defined as calmodulin (CAM) genes. Fifty additional genes are CAM-like (CML) genes, encoding proteins composed mostly of EF-hand Ca²⁺ -binding motifs, have no other identifiable functional domains, and at least 16% identical with CaM. Number and structural diversity of the EF hands are evaluated. Intron/exon boundaries, phylogenetic tree and chromosomal distribution data for the CAMs and CMLs are presented. •   Arabidopsis has 6 CAM genes, encoding only 3 isoforms. Maintenance of these genes suggests that they are unlikely to be fully redundant in function. Furthermore, the repeated EF hand motif is incorporated into at least 50 additional loci. The CaM relatives have altered EF hand number, organization, and predicted functional capacity. Additional structural differences and expression behaviors also indicate that the CML family has likely evolved distinct roles from the CAMs.