Heterocyst Development and Pattern Formation

Abstract

Most of the information about heterocyst development to date is based on the study of three species of heterocyst-forming filamentous cyanobacteria: Anabaena (also Nostoc) sp. strain PCC 7120, A. variabilis ATCC 29413, and Nostoc punctiforme ATCC 29133. This chapter focuses on those genes involved in signaling and regulation. HetR plays a central role in heterocyst development and pattern formation. In bacteria, calcium ions play important roles in various cellular processes such as pathogenesis, sporulation in Bacillus, chemotaxis in Escherichia coli, and heterocyst development in cyanobacteria. PatA may influence heterocyst development by attenuating the negative effects of the main inhibitory signals of heterocyst pattern formation, PatS and HetN. Late stages of heterocyst development are characterized by structural changes that include the deposition of three cell layers: an outermost fibrous layer, an envelope polysaccharide layer, and an innermost glycolipid layer. During nitrogen fixation, nitrogenase reduces atmospheric nitrogen to ammonia, which is then assimilated into amino acids. A recent epistasis analysis of four genes involved in pattern formation in Anabaena Strain PCC 7120 suggests that PatA has two distinct activities, to promote differentiation as well as to attenuate the negative effects of PatS and HetN on differentiation. Some genes that are required for heterocyst development are also involved in akinete formation, such as hetR and hepA. In the absence of heterocysts, the akinetes seem to form at random positions along the filament, whereas the presence of heterocysts influences akinete positioning, implying the existence of cell-to-cell communication.