Synechocystis sp. PCC6803 is a strain of unicellular, freshwater cyanobacteria. Synechocystis sp. PCC6803 is capable of both phototrophic growth by oxygenic photosynthesis during light periods and heterotrophic growth by glycolysis and oxidative phosphorylation during dark periods. Gene expression is regulated by a circadian clock and the organism can effectively anticipate transitions between the light and dark phases.
Evolutionary history
Cyanobacteria are photosynthetic prokaryotes that have existed on Earth for an estimated 2.7 billion years. The ability of cyanobacteria to produce oxygen initiated the transition from a planet consisting of high levels of carbon dioxide and little oxygen, to what has been called the Great Oxygenation Event where large amounts of oxygen gas were produced. Cyanobacteria have colonized a wide diversity of habitats, including fresh and salt water ecosystems, and most land environments. Phylogenetically, Synechocystis branches off later in the cyanobacterial evolutionary tree, further from the ancestral root (Gloeobacter violaceus). Synechocystis, which is non-diazotrophic, is closely related to another model organism, Cyanothece ATCC 51442, which is a diazotroph. Thus, it has been proposed that Synechocystis originally possessed the ability to fix nitrogen gas, but lost the genes required for a fully functioning nitrogen fixation (nif) gene cluster.
Growth and use as a model organism
Cyanobacteria are model microorganisms for the study of photosynthesis, carbon and nitrogen assimilation, evolution of plant plastids, and adaptability to environmental stresses. Synechocystis sp. PCC6803 is one of the most highly studied types of cyanobacteria as it can grow both autotrophically or heterotrophically in the absence of light. It was isolated from a freshwater lake in 1968 and grows best between 32 and 38 degrees Celsius. Synechocystis sp. PCC6803 can readily take up exogenous DNA, in addition to up taking DNA via electroporation, ultrasonic transformation and conjugation. The photosynthetic apparatus is very similar to the one found in land plants. The organism also exhibits phototactic movement.
Synechocystis sp. PCC6803 can be grown on either agar plates or in liquid culture. The most widely used culture medium is a BG-11 salt solution. The ideal pH is between 7 and 8.5. A light intensity of 50 μmol photons m−2 s−1 leads to best growth. Bubbling with carbon dioxide enriched air (1–2% CO2) can increase the growth rate, but may require additional buffer to maintain pH
Selection is typically performed by antibiotic resistance genes. Heidorn et al. 2011 experimentally determined in Synechocystis sp. PCC6803 the ideal concentrations of kanamycin, spectinomycin, streptomycin, chloramphenicol, erythromycin, and gentamicin. Cultures can be kept on agar plates for approximately 2 weeks and re-streaked indefinitely. For long term storage, liquid cell cultures should be stored in a 15% glycerol solution at -80 degrees Celsius.
Genome
The genome of Synechocystis sp. PCC6803 is contained within approximately 12 copies of a single chromosome (3.57 megabases), three small plasmids: pCC5.2 (5.2 kb) pCA2.4 (2.4 kb), and pCB2.4 (2.4 kb) and four large plasmids: pSYSM (120 kb), pSYSX (106 kb), pSYSA (103kb), and pSYSG (44 kb). The genome of Synechocystis sp. PCC6803 is the fourth genome to be completely sequenced, and the first phototrophic organism to have its genome fully sequenced.
Additional strains
The primary strain of Synechocystis sp. is PCC6803. Further modifications of the parent PCC6803 strain have been created, such as a sub-strain lacking photosystem 1 (PSI). The other widely used sub-strain of Synechocystis sp. is a glucose tolerant strain, ATCC 27184. The parent PCC 6803 strain cannot utilize external glucose.
Light-activated heterotrophy
Synechocystis sp. PCC6803, sub-strain ATCC 27184 can live heterotrophically in the dark on the carbon source glucose, but for yet unknown reasons requires a minimum of 5 to 15 minutes (blue) light per day. This regulatory role of light is intact in both PSI and PSII deficient strains.
Some glycolytic genes are regulated by the genesll1330 under light and glucose-supplemented conditions. One of the most important glycolytic genes is fructose-1,6-bisphosphate aldolase (fbaA). The mRNA level of fbaA is increased under light and glucose-supplemented conditions.