AbstractThe organismic unit of heterocyst-forming cyanobacteria is a filament of communicating cells connected by septal junctions, proteinaceous structures bridging the cytoplasms of contiguous cells. This distinct bacterial organization is preserved during cell division. In Anabaena, deletion of the zipN gene could not be segregated. We generated strain CSL109 that expresses zipN from a synthetic regulatable promoter. Under conditions of ZipN depletion, cells progressively enlarged, reflecting restricted cell division, and showed drastic morphological alterations including cell detachment from the filaments, to finish lysing. In contrast to the wild-type localization in midcell Z-rings, FtsZ was found in delocalized aggregates in strain CSL109. Consistently, the proportion of membrane-associated to soluble FtsZ in fractionated cell extracts was lower in CSL109. Bacterial two-hybrid analysis showed that ZipN interacts with FtsZ and other cell-division proteins including cytoplasmic Ftn6 and SepF, and polytopic FtsW, FtsX, FtsQ and FtsI. Additionally, ZipN interacted with the septal protein SepJ, and in CSL109 depletion of ZipN was concomitant with a progressive loss of septal specificity of SepJ. Thus, in Anabaena ZipN represents an essential FtsZ membrane tether and an organizer of the divisome, and it contributes to the conformation of septal structures for filament integrity and intercellular communication.