Expression of gfp-mut2 from the two promoters of the devB gene in wild-type, ntcA, and hetR backgrounds after N stepdown. GFP fluorescence (top) and bright-field (bottom) micrographs of filaments of strains CSSC6 (dual devB promoter directing expression of GFP in the wild-type background), CSSC7 (distal promoter in the wild-type background), CSSC8 (proximal promoter in the wild-type background), CSSC9 (dual promoter in the ntcA background), CSSC10 (distal promoter in the ntcA background), CSSC11 (proximal promoter in the ntcA background), CSSC12 (dual promoter in the hetR background), CSSC13 (distal promoter in the hetR background), and CSSC14 (proximal promoter in the hetR background), grown in shaken cultures with ammonium and incubated in the absence of combined nitrogen for 40 h. Settings to capture GFP fluorescence were identical only for the three strains reporting GFP from the same promoter fragment. Arrows point to some (pro)heterocysts. Size marker, 10 μm.