Biosynthetic pathway of mycosporines (mycosporine-like amino acids) in the cyanobacterium Chlorogloeopsis sp strain PCC 6912

We used metabolic inhibitors and radiolabelled tracers to investigate the biosynthetic pathway for mycosporines (mycosporine-like amino acids) in the facultatively heterotrophic cyanobacterium (blue-green alga) Chlorogloeopsis sp. strain PCC 6912. C-14-pyruvate was taken up and resulted in labelling of the core cyclohexenone, whereas feeding with C-14-acetate did not label cyclohexenone, in spite of strong cellular uptake. In addition, mycosporine synthesis was depressed in the presence of externally administered tyrosine (5 mM), a known feedback inhibitor of the shikimic acid pathway in cyanobacteria. We thus conclude that the mycosporine core cyclohexenone in cyanobacteria is derived from the shikimate pathway, as has been suggested for eukaryotic organisms. The use of other putative inhibitors did not yield clear evidence, but cerulenin was found to be a potent inhibitor of mycosporine biosynthesis. Using radiolabelled amino acids, we determined the origin of the variable mycosporine amino acidic side chains. We detected specific incorporation of C-14-glycine and C-14-serine into the corresponding side chains of mycosporine-glycine and shinorine, demonstrating that free amino acids are their direct precursors. A pulse-chase experiment using C-14-glycine demonstrated that the monosubstituted mycosporine, mycosporine-glycine, is a direct metabolic precursor of the bisubstituted mycosporine, shinorine. These results support the idea that the large diversity of mycosporine molecules found in nature is produced by variations in late biosynthetic steps, which add amino-moieties to the core. In particular, we can postulate the existence of a shinorine synthase catalysing the condensation of mycosporine-glycine and serine in this cyanobacterium.