A protein of cyAbrB family of regulators, Alr0946 (CalA), was identified by DNA affinity chromatography using DNA fragments from the promoter region of ftsZ in Anabaena/Nostoc sp. PCC 7120. Electrophoretic mobility shift assays showed that the protein could bind to upstream regions of ftsZ (as biotin-labeled DNA fragments) and that the binding was inhibited by the same DNA fragments and a fragment upstream of hetC but not by 3 fragments upstream of glnA, hetR or ntcA. Both transcriptional fusion with gfp (green fluorescence protein) and Western blot analysis indicated that the expression of calA was down-regulated in heterocysts relative to vegetative cells. An insertional mutant of calA could not be completely segregated. Over-expression of calA in Anabaena/Nostoc caused no change in growth, heterocyst differentiation or expression of ftsZ. CalA as a DNA-binding protein is essential for the growth of Anabaena/Nostoc and may be required for the expression of ftsZ in vegetative cells.