Tansley Review No. 107. Heterocyst and akinete differentiation in cyanobacteria

<jats:p>Summary 3</jats:p><jats:p>I. <jats:sc>introduction</jats:sc> 4</jats:p><jats:p>II. <jats:sc>the cyanobacteria</jats:sc> 7</jats:p><jats:p>III. <jats:sc>the heterocyst</jats:sc> 9</jats:p><jats:p>1. <jats:italic>Function and metabolism</jats:italic> 9</jats:p><jats:p>2. <jats:italic>Heterocyst structure</jats:italic> 12</jats:p><jats:p>(a) <jats:italic>Overview</jats:italic> 12</jats:p><jats:p>(b) <jats:italic>The polysaccharide</jats:italic> (<jats:italic>homogeneous</jats:italic>) <jats:italic>layer</jats:italic> 12</jats:p><jats:p>(c) <jats:italic>The glycolipid</jats:italic> (<jats:italic>laminated</jats:italic>) <jats:italic>layer</jats:italic> 12</jats:p><jats:p>(d) <jats:italic>The septum and microplasmodesmata</jats:italic> 12</jats:p><jats:p>3. <jats:italic>Nitrogen regulation and heterocyst development</jats:italic> 12</jats:p><jats:p>4. <jats:italic>Heterocyst development</jats:italic> 13</jats:p><jats:p>(a) <jats:italic>The proheterocyst</jats:italic> 13</jats:p><jats:p>(b) <jats:italic>Proteolysis associated with heterocyst development</jats:italic> 14</jats:p><jats:p>(c) <jats:italic>RNA polymerase sigma factors</jats:italic> 14</jats:p><jats:p>(d) <jats:italic>Developmental regulation of heterocyst cell wall and nitrogenase gene expression</jats:italic> 14</jats:p><jats:p>(e) <jats:italic>Genome rearrangements associated with heterocyst development</jats:italic> 15</jats:p><jats:p>5. <jats:italic>Genes essential for heterocyst development</jats:italic> 15</jats:p><jats:p>(a) <jats:italic>hetR</jats:italic> 15</jats:p><jats:p>(b) <jats:italic>Protein phosphorylation and the regulation of</jats:italic> hetR <jats:italic>activity</jats:italic> 16</jats:p><jats:p>(c) hetR <jats:italic>in nonheterocystous cyanobacteria</jats:italic> 16</jats:p><jats:p>(d) <jats:italic>Other heterocyst‐specific genes</jats:italic> 16</jats:p><jats:p>6. <jats:italic>Heterocyst spacing</jats:italic> 18</jats:p><jats:p>(a) <jats:italic>Patterns of heterocyst differentiation</jats:italic> 18</jats:p><jats:p>(b) <jats:italic>Genes involved in heterocyst spacing</jats:italic> 18</jats:p><jats:p>(c) <jats:italic>Disruption of heterocyst pattern</jats:italic> 18</jats:p><jats:p>7. <jats:italic>Filament fragmentation and the regression of developing heterocysts</jats:italic> 20</jats:p><jats:p>8. <jats:italic>The nature of the heterocyst inhibitor</jats:italic> 20</jats:p><jats:p>9. <jats:italic>Cell selection during differentiation and pattern formation</jats:italic> 20</jats:p><jats:p>(a) <jats:italic>Cell division</jats:italic> 20</jats:p><jats:p>(b) <jats:italic>DNA replication and the cell cycle</jats:italic> 21</jats:p><jats:p>(c) <jats:italic>Competition</jats:italic> 21</jats:p><jats:p>10. <jats:italic>Models for heterocyst differentiation and pattern control</jats:italic> 21</jats:p><jats:p>IV. <jats:sc>the akinete</jats:sc> 23</jats:p><jats:p>1. <jats:italic>Properties of akinetes</jats:italic> 23</jats:p><jats:p>2. <jats:italic>Structure, composition and metabolism</jats:italic> 24</jats:p><jats:p>3. <jats:italic>Relationship to heterocysts</jats:italic> 24</jats:p><jats:p>4. <jats:italic>Factors that influence akinete differentiation</jats:italic> 24</jats:p><jats:p>5. <jats:italic>Extracellular signals</jats:italic> 25</jats:p><jats:p>6. <jats:italic>Akinete germination</jats:italic> 25</jats:p><jats:p>7. <jats:italic>Genes involved in akinete differentiation</jats:italic> 26</jats:p><jats:p>V. <jats:sc>conclusion</jats:sc> 26</jats:p><jats:p>Acknowledgements 27</jats:p><jats:p>References 28</jats:p><jats:p>Cyanobacteria 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.</jats:p>