Signal Transduction in Heterocyst Differentiation of the Cyanobacterium Anabaena Sp. PCC 7120

Abstract

The cyanobacteria are a group of gram-negative bacteria that perform oxygenic photosynthesis. Some filamentous cyanobacteria are able to form specialized cells, called heterocysts, specialized in nitrogen fixation under nitrogen limiting growth conditions (Adam, 2000). Heterocyst properties allow them to maintain a low internal oxygen concentration for optimal nitrogenase activity (Wolk et al., 1994). Heterocysts have a thick envelope consisting of a polysaccharide layer and a glycolipid layer that limits oxygen penetration. Heterocysts lack photosystem II and do not evolve oxygen. A high respiratory activity of heterocyst helps consuming oxygen inside the cells. Heterocysts also lack Rubisco and cannot fix CO2. The reducing power for nitrogenase comes from sugar supply from adjacent vegetative cells. In return, heterocysts provide fixed nitrogen to vegetative cells probably in the form of amino acids. Because the metabolite exchange between vegetative cells and heterocysts is required for a normal function of heterocysts, chains of heterocysts are rarely seen on cyanobacterial filaments. In fact, heterocyst distribution along the filaments is often present in a semi-regular pattern. In Anabaena sp. PCC 7120, there is one heterocyst per approximately 10-15 vegetative cells. According to fossil record, heterocyst might the earliest example of differentiation in biological world (Gobulic et al, 1995).