Scytonemin is a promising UV-screen and antioxidant small molecule which is widely used commercially in cosmetics and medicine. It is exclusively biosynthesized in some cyanobacteria. Recently, its biosynthesis mechanism has been explained in the model cyanobacterium Nostocpunctiforme PCC 73102. 

Tryptophan and p-hydroxyphenylpyruvate are the direct precursors for scytonemin biosynthesis. They are generated through the shikimate and aromatic amino acid biosynthesis pathways. More upstream substrates serve as the central carbon metabolism intermediates phosphoenolpyruvate and erythrose-4-phosphate. Thus, a long route is employed to synthesize scytonemin from the fixed atmospheric CO2 in cyanobacteria. 

Metabolic engineering is emerging as an important biotechnological means for achieving sustainable high-efficiency and high-yield target metabolites. Since Scytonemin has a high potential market value, Metabolic engineering techniques will provide one of the key solutions for achieving its cost-effective production in the near future. The previous engineering in the E. coli strain only produced the scytonemin monomer. Recent explanation of its biosynthesis mechanism and related carbon flux-rewiring technologies, highly efficient biosynthesis of scytonemin in suitable cyanobacterial hosts, including N. flagelliforme and N. punctiforme, is anticipated to be an important starting point.

Source- Gao X, Jing X, Liu X, Lindblad P. Biotechnological Production of the Sunscreen Pigment Scytonemin in Cyanobacteria: Progress and Strategy. Mar Drugs. 2021;19(3):129. Published 2021 Feb 27. doi:10.3390/md19030129