Compartmentalized function through cell differentiation in filamentous cyanobacteria

Key Points Filamentous cyanobacteria such as Anabaena and Nostoc are oxygenic photoautotrophs that grow as trichomes in which some cells can differentiate to become specialized in different functions, producing a multicellular organism. Cyanobacteria bear a Gram-negative type of cell envelope, carrying an outer membrane outside of the cytoplasmic membrane and murein sacculus. In filamentous cyanobacteria, whereas the cytoplasmic membrane and murein sacculus surround each cell, the outer membrane is continuous along the filament, defining a continuous periplasmic space that could represent a communication conduit between cells. At the intercellular septa, protein complexes, of which SepJ (also known as Alr2338) is an identified component, help to keep cells together in the filament and might also have a role in intercellular communication, which can be probed with a fluorescent tracer, such as calcein. In response to the environmental cue of nitrogen deficiency, some cells positioned at semi-regular intervals in the filament differentiate into N 2 -fixing heterocysts, a process that involves a specific programme of gene expression. Two proteins that are key to this process are the cyanobacterial nitrogen control transcription factor NtcA and heterocyst differentiation control protein (HetR), which are needed for differentiation to take place. Differentiation of too many vegetative cells into heterocysts is prevented by inhibitors such as the small, diffusible heterocyst inhibition-signalling peptide (PatS) or a PatS-related compound, which are produced by the differentiating heterocysts. The developed Anabaena filament containing two types of interdependent cells, the CO 2 -fixing vegetative cells and the N 2 -fixing heterocysts, is a fascinating case of multicellularity in the bacterial world that we are only starting to understand. In this Review, Flores and Herrero describe how some cyanobacteria form multicellular filaments containing cells that are differentiated to carry out specialized functions. This compartmentalization allows the bacteria to overcome the problems that are associated with incompatible metabolic functions such as oxygenic photosynthesis and N 2 fixation. Within the wide biodiversity that is found in the bacterial world, Cyanobacteria represents a unique phylogenetic group that is responsible for a key metabolic process in the biosphere — oxygenic photosynthesis — and that includes representatives exhibiting complex morphologies. Man