Abstract
Summary 3I. introduction 4II. the cyanobacteria 7III. the heterocyst 91. Function and metabolism 92. Heterocyst structure 12(a) Overview 12(b) The polysaccharide (homogeneous) layer 12(c) The glycolipid (laminated) layer 12(d) The septum and microplasmodesmata 123. Nitrogen regulation and heterocyst development 124. Heterocyst development 13(a) The proheterocyst 13(b) Proteolysis associated with heterocyst development 14(c) RNA polymerase sigma factors 14(d) Developmental regulation of heterocyst cell wall and nitrogenase gene expression 14(e) Genome rearrangements associated with heterocyst development 155. Genes essential for heterocyst development 15(a) hetR 15(b) Protein phosphorylation and the regulation of hetR activity 16(c) hetR in nonheterocystous cyanobacteria 16(d) Other heterocyst‐specific genes 166. Heterocyst spacing 18(a) Patterns of heterocyst differentiation 18(b) Genes involved in heterocyst spacing 18(c) Disruption of heterocyst pattern 187. Filament fragmentation and the regression of developing heterocysts 208. The nature of the heterocyst inhibitor 209. Cell selection during differentiation and pattern formation 20(a) Cell division 20(b) DNA replication and the cell cycle 21(c) Competition 2110. Models for heterocyst differentiation and pattern control 21IV. the akinete 231. Properties of akinetes 232. Structure, composition and metabolism 243. Relationship to heterocysts 244. Factors that influence akinete differentiation 245. Extracellular signals 256. Akinete germination 257. Genes involved in akinete differentiation 26V. conclusion 26Acknowledgements 27References 28Cyanobacteria are an ancient and morphologically diverse group of photosynthetic prokaryotes. They were the first organisms to evolve oxygenic photosynthesis, and so changed the Earth's atmosphere from anoxic to oxic. As a consequence, many nitrogen‐fixing bacteria became confined to suitable anoxic environmental niches, because the enzyme nitrogenase is highly sensitive to oxygen. However, in the cyanobacteria a number of strategies evolved that protected nitrogenase from oxygen, including a temporal separation of oxygenic photosynthesis and nitrogen fixation and, in some filamentous strains, the differentiation of a specialized cell, the heterocyst, which provided a suitable microaerobic environment for the functioning of nitrogenase. The evolution of a spore‐like cell, the akinete, almost certainly preceded that of the heterocyst and, indeed, the akinete may have been the ancestor of the heterocyst. Cyanobacteria have the capacity to differentiate several additional cell and filament types, but this review will concentrate on the heterocyst and the akinete, emphasizing the differentiation and spacing of these specialized cells.
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