Cyanobacterial Respiration.

Cyanobacterial respiration has some unique features. Cyanobacteria contain two independent respiratory chains, one in the cytoplasmic membrane and one in the intracytoplasmic membranes (also called thylakoids). The latter is intimately linked with the photosynthetic electron transport chain and some components, e.g. the quinone pool, the cytochrome b6f complex, and cytochrome c6, are shared by both photosynthesis and respiration. However, all components of the respiratory electron transport chain might have some—possibly regulatory—function in photosynthesis. In recent years the total genomic sequences of several cyanobacteria have been invaluable tools for understanding cyanobacterial respiration. Especially in Synechocystis sp. Strain PCC 6803 the majority of the components of the respiratory chains were finallly identified. Respiratory electron transport is highly branched, with several routes of electron input and several routes to the final electron acceptor, which is always dioxygen. The single component common to all respiratory electron pathways is the quinone pool. Furthermore, total genomic sequencing has shown that different cyanobacteria may contain different elecron transport components. Several strains, especially (but not exclusively) those that are capable of cell differentiation, contain more than one copy of genes for some respiratory electron transport chain components. The different functions of these genes and their gene products have only partially been elucidated. Several components of the respiratory chains are subject to complex gene regulations, about which information is only starting to appear. Some proteins, e.g. cytochrome cM, and the dehydrogenase DrgA, are likely to be involved in respiratory electron transport, but their positions in the electron transport chain (i.e. their in vivo electron donors and acceptors) have not yet been fully clarified. The electron transport chain of the cytoplasmic membrane is not yet well characterized in any strain. [ABSTRACT FROM AUTHOR]