Cyanobacterial Respiration

Abstract

Cyanobacterial respiration is unique in several respects. Oxygenic photosynthesis and aerobic respiration are not separated in different organelles as in plants, but are active in the same compartment. Moreover, at least some cyanobacteria contain two distinct and complete respiratory chains, with one being found in each of their bioenergetically active membranes: the cell membrane and the intracellular membranes or thylakoids. In contrast, photosynthetic electron transport is generally present only in the intracellular membranes. Components not involved in respiration in other organisms are parts of the respiratory electron transport chain, such as NADPH, plastoquinone, and a chloroplast-type cytochrome b 6 f complex. Several structural genes for components of the respiratory chain have been cloned. Even in those cyanobacteria that contain two distinct respiratory chains, almost all genes are present in only one copy per chromosome. How one gene directs its gene product into two different membranes is an intriguing yet unanswered question. The amounts of several components of the respiratory chain are regulated by external conditions: e.g., cytochrome c 553 and plastocyanin by the concentration of Cu 2+ and aa 3-type cytochrome oxidase by the ionic strength of the medium and (in heterocyst forming strains) by the availability of combined nitrogen. The elucidation of these mechanisms of gene regulation, largely by applying the methods of molecular biology to cyanobacteria, will be among the topics of the future studies in cyanobacterial respiration.