30.07.2011 14:00 Uhr
Cyanobacteria are a group of prokaryotic organisms characterized by their ability to fix CO2 by oxygenic photosynthesis. They are considered the ancestors of the chloroplasts and the inventors of oxygenic photosynthesis, and are among the most important primary producers of the planet. They represent a phylogenetically coherent group, but show a very diverse morphology and have colonized a wide diversity of habitats.
Cyanobacteria are Gram-negative bacteria and all bear a cell envelope architecture consisting of an inner membrane and an outer membrane separated by a periplasmic space that contains a peptidoglican layer. This sophisticated and complex cell envelope protects the cells and hosts special lipopolysaccharides (LPS) and integral membrane proteins which serve essential functions for the cell, such as nutrient uptake, cell adhesion, cell signaling and waste export. To fulfill these functions, the cell envelope requires the assistance of distinct trafficking complexes and assembly machineries to correctly deliver and insert α‑helical membrane proteins in the inner membrane and β‑barrel membrane proteins and lipopolysaccharides in the outer membrane, being the regulation of these machineries an essential process.
Filamentous cyanobacteria species are true multicellular organisms in the form of filaments of cells that show a common and continuous outer membrane and periplasm along the filament, making them a unique group of prokaryotes. Cells in the filaments communicate between each other and, in heterocyst-forming cyanobacteria such as Anabaena sp. PCC 7120, show a division of labors in different cell types. Under combined nitrogen deprivation, heterocyst-forming cyanobacteria present vegetative cells which perform oxygenic photosynthesis and heterocysts which carry out N2 fixation and do not perform concomitant fixation of CO2. These specialized cells rely on each other: heterocysts require photosynthate that is provided by vegetative cells, and heterocysts in turn provide vegetative cells with fixed nitrogen.
Nearly half of all enzymes in organisms require metals such as Mg, Zn, Fe, Mn, Ca, Cu, Co and Ni (in order of frequency), so the regulation of metal availability is a key factor and cells control the concentration of each metal in the cytosol and the periplasm through the combined actions of proteins of metal homeostasis including importers, exporters, storage proteins, delivery proteins and sensors. Cyanobacteria have high metal demands to support oxygenic photosynthesis and other metabolic activities and, in the case of iron, require an iron quota ten times higher than that of Escherichia coli. However, iron bioavailability is very low and some cyanobacteria secrete the strongest iron chelators in nature, known as siderophores, to fulfill their high iron uptake demands. Siderophores show a wide diversity of structures and can be molecules based on citric acid or peptides. The first group utilizes specific enzymes involved exclusively in these biosynthetic routes, while the second group is directed by a large family of modular enzymes called non-ribosomal peptide synthetases, which are also the cell factories for the biosynthesis of the majority of the microbial peptide secondary metabolites, such as peptide antibiotics and toxins, and are a newly discovered alternative machinery to ribosomes for the synthesis of some peptides with a huge potential for the development of new compounds and drugs.
The model organism used in our laboratory is the filamentous heterocyst-forming cyanobacterium Anabaena sp. PCC 7120, and our research is focused on understanding membrane biogenesis, trafficking through membranes and the intercellular relationships in cyanobacterial filaments; metal transport and regulation processes; and non-ribosomal peptide synthetases involved in the biosynthesis of siderophores and other secondary metabolites, such as antibiotics. All these topics are fundamental to understand the biology and ecology of these important organisms for the life of the planet, and to provide new insights in some essential processes in cells.