Heterocyst Envelope Glycolipids.

Heterocyst-forming cyanobacteria simultaneously photosynthesize, producing oxygen (O₂), and fix dini-trogen (N₂), initially into ammonia, using nitrogenase enzymes that are rapidly inactivated by O₂. These cyanobacteria enable nitrogenases to function in an oxic environment by segregating them within specialized cells, called heterocysts, in which O₂ is not produced, respiration is highly active, and an envelope barrier of glycolipids greatly slows the rate of entry of O₂. We will describe the chemical structure of the heterocyst-specific glycolipids (Hgls), their physiological role, and what is known of their deposition. We will then discuss the clustered genes that encode the proteins required for their biosynthesis, how the glycolipids are believed to be synthesized, and what is known of the regulation of their biosynthesis. Finally, we will examine the relationship between their biosynthetic enzymes and other polyketide synthases, with an emphasis on those from cyanobacteria. [ABSTRACT FROM AUTHOR]