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Bacterial sunscreens
In order to access the solar radiation required for photosynthesis, cyanobacteria must live in environments exposed to some level of solar ultra violet radiation or UVR (wavelengths shorter than 400 nm), which is associated with biologically deleterious effects. UV-A (315-400 nm) causes long-term damage through oxidizing agents. Despite the hazardous effects of solar UVR, cyanobacteria continue to thrive in habitats exposed to high doses of UVR such as soil and rock surfaces, and thus have various UV defense mechanisms such as the ability to synthesize and accumulate UV-sunscreens.
Sunscreens serve as passive preventative mechanisms that allow the organism to stop UVR before it reaches the cellular machinery, DNA, or produces reactive oxygen species. The indole-alkaloid, scytonemin, is one such sunscreen, found exclusively among cyanobacteria. It is a brownish-yellow, lipid-soluble pigment located in the extracellular matrix of the cells.
The production of scytonemin is induced by UV-A and the conjugated double-bond distribution allows for the molecule to absorb strongly in that range (with a maximum of ~384 nm), effectively absorbing ambient UV irradiation before it can reach the cell. Scytonemin also has potential in biomedical applications because of its strong anti-proliferative and anti-inflammatory activity. Another class of sunscreens found in cyanobacteria are the mycosporine-like amino acids (MAAs) which absorb and are induced by UV-B (280-315 nm).
Our lab is currently focusing on the molecular genetics of scytonemin biosynthesis.
A Nostoc ATCC 29133 colony grown under relatively high light conditions. Note that the colony is fairly bleached but no scytonemin is produced. (Viewed under a dissecting microscope)
A Nostoc ATCC 29133 colony grown under UV-A stress. Note the copper color of the colony signifying scytonemin production. (Viewed under a dissecting microscope)
A typical filament of Nostoc ATCC 29133. Note the extracellular polysaccharides are clear in color meaning no extracellular pigment has been produced. (Viewed under a phase contrast microscope)
A knot of filament of Nostoc ATCC 29133. Note the extracellular polysaccharides are copper/gold in color showing the presence of the pigment scytonemin. (Viewed under a phase contrast microscope)
The model organism used in our lab to study sunscreen biosynthesis is the cyanobacterium Nostoc punctiforme ATCC 29133 (PCC 73102). It is a heterocystous, filamentous cyanobacterium in the order Nostocales.
N. punctiforme was originally isolated from the symbiotic association with the gymnosperm cycad Macrozamia sp. Its genome is fully sequenced and available in the US Department of Energy's Joint Genome Institute (JGI) database . N. punctiforme's genome is approximately 9.05 Mb (41.4% GC) and contains approximately 7364 putative gene encoding ORFs. Using N. punctiforme as a model organism we have been able to obtain a scytonemin-deficient mutant by random transposon insertion into a putative gene. The genomic region of mutation has been identified and is currently being studied for its significance in the biosynthesis of scytonemin as well as its presence in other scytonemin-producing cyanobacteria.
N. punctiforme grown under white light without scytonemin (left panel), and under UV-A irradiation with scytonemin indicated by the dark brown pigmentation (right panel).