THE STRUCTURE OF SCYTONEMIN, AN ULTRAVIOLET SUNSCREEN PIGMENT FROM THE SHEATHS OF CYANOBACTERIA

Despite knowledge of the existence of the pigment called scytonemin fo r over 100 years, its structure has remained unsolved until now. This pigment, the first shown to be an effective, photo-stable ultraviolet shield in prokaryotes, is a novel dimeric molecule (molec. wt. 544) of indolic and phenolic subunits and is known only from the sheaths encl osing the cells of cyanobacteria. It is probable that scytonemin is fo rmed from a condensation of tryptophan- and phenylpropanoid-derived su bunits. The linkage between these units is unique among natural produc ts and this novel ring structure is here termed the 'scytoneman skelet on'. Scytonemin absorbs strongly and broadly in the spectral region 32 5-425 nm (UV-A-violet-blue, with an in vivo maximum at 370 nm). Howeve r, there is also major absorption in the UV-C (lambda(max) = 250 nm) a nd UV-B (280-320 nm). The pigment has been recently shown to provide s ignificant protection to cyanobacteria against damage by ultraviolet r adiation. The pigment occurs in all phylogenetic lines of sheathed cya nobacteria and possibly represents a UV screening strategy far more an cient than that of plant flavonoids and animal melanins. How diverse o rganisms deal with UV radiation is considered of vital importance to g lobal ecology.