Abstract
Cyanobacteria are photosynthetic organisms with a Gram-negative envelope structure. Certain filamentous species such as Anabaena sp. strain PCC 7120 can fix dinitrogen upon depletion of combined nitrogen. Because the nitrogen-fixing enzyme, nitrogenase, is oxygen sensitive, photosynthesis and nitrogen fixation are spatially separated in Anabaena. Nitrogen fixation takes place in specialized cells called heterocysts, which differentiate from vegetative cells. During heterocyst differentiation, a microoxic environment is created by dismantling photosystem II and restructuring the cell wall. Moreover, solute exchange between the different cell types is regulated to limit oxygen influx into the heterocyst. The septal zone containing nanopores for solute exchange is constricted between heterocysts and vegetative cells, and cyanophycin plugs are located at the heterocyst poles. We identified a protein previously annotated as TonB1 that is largely conserved among cyanobacteria. A mutant of the encoding gene formed heterocysts but was impaired in diazotrophic growth. Mutant heterocysts appeared elongated and exhibited abnormal morphological features, including a reduced cyanophycin plug, an enhanced septum size, and a restricted nanopore zone in the septum. In spite of this, the intercellular transfer velocity of the fluorescent marker calcein was increased in the mutant compared to the wild type. Thus, the protein is required for proper formation of septal structures, expanding our emerging understanding of Anabaena peptidoglycan plasticity and intercellular solute exchange, and is therefore renamed SjdR (septal junction disk regulator). Notably, calcium supplementation compensated for the impaired diazotrophic growth and alterations in septal peptidoglycan in the sjdR mutant, emphasizing the importance of calcium for cell wall structure. IMPORTANCE Multicellularity in bacteria confers an improved adaptive capacity to environmental conditions and stresses. This includes an enhanced capability of resource utilization through a distribution of biochemical processes between constituent cells. This specialization results in a mutual dependency of different cell types, as is the case for nitrogen-fixing heterocysts and photosynthetically active vegetative cells in Anabaena. In this cyanobacterium, intercellular solute exchange is facilitated through nanopores in the peptidoglycan between adjacent cells. To ensure functionality of the specialized cells, septal size as well as the position, size, and frequency of nanopores in the septum need to be tightly established. The novel septal junction disk regulator SjdR characterized here is conserved in the cyanobacterial phylum. It influences septal size and septal nanopore distribution. Consequently, its absence severely affects the intercellular communication and the strains' growth capacity under nitrogen depletion. Thus, SjdR is involved in septal structure remodeling in cyanobacteria.