Cyanobacteria developed oxygenic photosynthesis and represent the phylogenetic ancestors of chloroplasts. The model strain Anabaena sp. PCC 7120 grows as filaments of communicating cells and can form heterocysts, cells specialized for N ₂ fixation. In the Anabaena genome, ORF all2390 is annotated as encoding a SulA homolog, but sequence similarity to SulA of model bacteria is unsignificant. We generated strains that lacked or overexpressed all2390, both of which showed instances of increased cell size, and observed that purified All2390 protein interfered with the in vitro polymerization of FtsZ. Heterocyst frequency diminished by all2390 inactivation and increased by all2390 overexpression. Overexpression retarded the dismantlement of Z-ring structures that sets commitment in the differentiating cells. Thus, All2390 can influence cell division affecting heterocyst differentiation. An All2390-GFP fusion protein localized to the thylakoidal membranes in vegetative cells and to the honeycomb membranes, which harbor photosynthetic complexes, in the heterocyst polar regions. Notably, all2390 expression strongly increased under high light, conditions under which the null mutant is unable to survive. Thus, All2390 appears essential for adaptation to high light conditions. We name All2390 ThyZ to reflect its dual role in FtsZ-ring dynamics and acclimation of thylakoidal membranes to increased light intensity.