FraC/FraD-dependent intercellular molecular exchange in the filaments of a heterocyst-forming cyanobacterium, Anabaena sp.

The filamentous, heterocyst-forming cyanobacteria are multicellular organisms in which two different cell types, the CO2-fixing vegetative cells and the N-2-fixing heterocysts, exchange nutrients and regulators. In Anabaena sp. strain PCC 7120, inactivation of sepJ or genes in the fraC operon (fraC, fraD and fraE) produce filament fragmentation. SepJ, FraC and FraD are cytoplasmic membrane proteins located in the filament's intercellular septa that are needed for intercellular exchange of the fluorescent tracer calcein (622 Da). Transmission electron microscopy showed an alteration in the heterocyst cytoplasmic membrane at the vegetative cell-heterocyst septa in Delta fraC and Delta fraD mutants. Immunogold labelling of FraD confirmed its localization in the intercellular septa and clearly showed the presence of part of the protein between the cytoplasmic membranes of the adjacent cells. This localization seemed to be affected in the Delta fraC mutant but was not impaired in a Delta sepJ mutant. Intercellular transfer of a smaller fluorescent tracer, 5-carboxyfluorescein (374 Da), was largely impaired in Delta fraC, Delta fraD and double Delta fraC-Delta fraD mutants, but much less in the Delta sepJ mutant. These results show the existence in the Anabaena filaments of a FraC/FraD-dependent intercellular molecular exchange that does not require SepJ.