Mycosporine-like amino acids (MAAs) are water-soluble pigments that absorb UV radiation of 280–340 nm. There are structurally distinct MAAs found in taxonomically diverse organisms. In cyanobacteria, MAAs protect the cells against solar radiation. The energy absorbed by MAAs is promptly dispersed into the surroundings as heat. Thus, MAAs do not function as accessory pigments in photosynthesis. It is noteworthy that all MAAs identified thus far in the terrestrial cyanobacterium Nostoc commune are glycosylated. The glycosylated MAAs in N. commune are thought to be localized in the extracellular matrix to allow interaction with other constituents in its complex architecture. These compounds have multiple functions including roles as UV sunscreens and radical scavengers to protect the cells in terrestrial environments where they are exposed to severe UV radiation, extreme temperature and desiccation. In this study, we examined the genotype A of N. commune colonies which its water extract showed a UV-absorbing spectrum with an absorption maximum at 335 nm. The water extract contained the following compounds: 7-O-(β-rabinopyranosyl)- porphyra-334 (478 Da), pentose-bound shinorine (464 Da), hexose-bound porphyra-334 (508 Da) and porphyra-334 (346 Da).According to the MAA contents and their structures, a MAA biosynthetic pathway is predicted in N. commune. In our predicted pathway, porphyra-334 and shinorine could be generated via the addition of threonine and serine, respectively, to the mycosporine–glycine intermediate. The following glycosylation of porphyra-334 and shinorine could produce the 478-Da MAA (as a main MAA), the 508-Da MAA and the 464-Da MAA. In the proposed biosynthetic pathway, the glycosylation used to produce the pentose-bound porphyra-334 and shinorine derivatives is a unique process in genotype A of N. commune. Additional molecular genetic studies to identify the genes involved in the glycosylation will elucidate the biosynthesis of the glycosylated MA
