Fra proteins influencing filament integrity, diazotrophy and localization of septal protein SepJ in the heterocyst-forming cyanobacterium Anabaena sp.

Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium that can fix N(2) in differentiated cells called heterocysts, which exchange nutritional and regulatory compounds with the neighbouring photosynthetic vegetative cells. The cells in the filament appear to be joined by some protein structures, of which SepJ (FraG) that is located at the cell poles in the intercellular septa and is needed for filament integrity seems to be a component. Other known proteins required for filament integrity include FraC and FraH. Whereas fraC (alr2392) was constitutively expressed as an operon together with two downstream genes, alr2393 (fraD) and alr2394 (fraE), fraH (alr1603) was induced under nitrogen deprivation. Single mutants of these genes showed filament fragmentation under nitrogen deprivation and did not grow diazotrophically, although they formed heterocysts. The fraC and fraD mutants showed an impaired localization of SepJ at the intercellular septa and were hampered in the intercellular transfer of the fluorescent probe calcein. As shown with GFP fusions, FraC and FraD are also located at the intercellular septa. Therefore, at least three different proteins, SepJ, FraC and FraD, influence the architecture and function of the intercellular septa in the Anabaena filaments.