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ARC5 is a dynamin-related GTPase essential for the division of chloroplasts in plants. The arc5 mutant frequently exhibits enlarged, dumbbell-shaped chloroplasts, indicating a role for ARC5 in the constriction of the chloroplast division site. In a screen for chloroplast division mutants with a phenotype similar to arc5, two mutants, cpd25 and cpd45, were obtained. CPD45 was identified as being the same gene as FHY3, a key regulator of far-red light signaling recently shown to be involved in the regulation of ARC5. CPD25 was previously named FRS4 and is homologous to FHY3. We found that CPD25 is also required for the expression of ARC5, suggesting that its function is not redundant to that of FHY3. Moreover, cpd25 does not have the far-red light-sensing defect present in fhy3 and far1. Both FRS4/ CPD25 and FHY3/ CPD45 could bind to the FBS-like ' ACGCGC' motifs in the promoter region of ARC5, and the binding efficiency of FRS4/ CPD25 was much higher than that of FHY3/ CPD45. Unlike FHY3/ CPD45, FRS4/ CPD25 has no ARC5 activation activity. Our data suggest that FRS4/ CPD25 and FHY3/ CPD45 function as a heterodimer that cooperatively activates ARC5, that FRS4/ CPD25 plays the major role in promoter binding, and that FHY3/ CPD45 is largely responsible for the gene activation. This study not only provides insight into the mechanisms underlying the regulation of chloroplast division in higher plants, but also suggests a model that shows how members of a transcription factor family can evolve to have different DNA-binding and gene activation features. [ABSTRACT FROM AUTHOR]