Functional characterization of the <i>mys</i> genes for porphyra-334 biosynthesis from the terrestrial cyanobacterium <i>Nostoc commune</i> by heterologous expression

Mycosporine-like amino acids (MAAs) are low-molecular-weight UV-protective compounds, and porphyra-334 and shinorine are common MAAs. Porphyra-334 is synthesized via the conjugation of mycosporine-glycine with threonine, whereas substitution with serine yields shinorine. The terrestrial cyanobacterium Nostoc commune KU002 (NIES-2538) produces 7-O-(β-arabinopyranosyl)-porphyra-334, and the mysABCD gene cluster responsible for MAA biosynthesis has been isolated. The heterologous expression of the mysABC genes from N. commune KU002 in Escherichia coli cells led to mycosporine-glycine production regardless of the culture medium supplemented with serine, threonine, or xylose. When the mysABCD genes from N. commune KU002 were expressed in E. coli cells, porphyra-334 production occurred, and shinorine production was observed upon serine supplementation in the culture medium. Notably, threonine and xylose supplementation in the culture medium increased the amounts of porphyra-334 in both cellular extracts and culture medium extracts. When the mysD gene was replaced with that from the shinorine producer Actinosynnema mirum JCM 3225, shinorine was primarily synthesized instead of porphyra-334. Interestingly, the transformant expressing the chimeric KU002-mysABC-JCM3225-mysD produced a novel MAA derivative with an absorption maximum at 334 nm and a molecular mass of 346 when cultured in the medium supplemented with threonine and xylose. These results suggest that the substrate specificity of MysD, which catalyzes the conjugation of mycosporine-glycine and serine or threonine, alters the production of porphyra-334 or shinorine and that the supplements added to the culture medium affect the amount and composition of MAAs produced in the E. coli transformant.