Biotechnological Production of the Sunscreen Pigment Scytonemin in Cyanobacteria: Progress and Strategy

Abstract

Scytonemin is a promising UV-screen and antioxidant small molecule with commercial value in cosmetics and medicine. It is solely biosynthesized in some cyanobacteria. Recently, its biosynthesis mechanism has been elucidated in the model cyanobacterium Nostoc punctiforme PCC 73102. The direct precursors for scytonemin biosynthesis are tryptophan and p-hydroxyphenylpyruvate, which are generated through the shikimate and aromatic amino acid biosynthesis pathway. More upstream substrates are the central carbon metabolism intermediates phosphoenolpyruvate and erythrose-4-phosphate. Thus, it is a long route to synthesize scytonemin from the fixed atmospheric CO₂ in cyanobacteria. Metabolic engineering has risen as an important biotechnological means for achieving sustainable high-efficiency and high-yield target metabolites. In this review, we summarized the biochemical properties of this molecule, its biosynthetic gene clusters and transcriptional regulations, the associated carbon flux-driving progresses, and the host selection and biosynthetic strategies, with the aim to expand our understanding on engineering suitable cyanobacteria for cost-effective production of scytonemin in future practices.

Keywords: cyanobacteria; metabolic engineering; scytonemin; sunscreen pigments.

Figure 1

The scytonemin molecule (A) and its 3D model (B). Two images are generated by the MolView program (https://molview.org/, accessed on 20 August 2020).

Figure 2

The filamentous colonies (filaments) of Nostoc flagelliforme in native habitats (A) and the microscopic observation of a crushed filament (B). Yellow-brown scytonemin is distributed throughout a filament as shown in (B).

Figure 3

Homologous comparison of scytonemin biosynthesis-related genes between two genetically close Nostoc species (A) and schematic illustration of UV-induced scytonemin biosynthesis processes in cyanobacteria (B). Npu, Nostoc punctiforme; Nfl, Nostoc flagelliforme. The gene clusters are distinguished as four modules. Module I, scyABCDEF, responsible for scytonemin monomer and dimer biosynthesis. Module II, tyrA~aroG, responsible for biosynthesis of aromatic amino acid substrates. Module III, eboFECBA, responsible for the export of scytonemin monomer. Module IV, responsible for signal transduction in response to UV radiation. Abbreviations: IM, inner membrane; OM, outer membrane; EPS matrix, exopolysaccharide matrix.

Figure 4

The transcriptional changes of scytonemin biosynthetic and export-associated genes in response to UV-B radiation. The physiologically recovered Nostoc flagelliforme samples were subjected to 0.5 W·m⁻² UV-B radiation for 24 h. (A) The basal RPKM values of the genes according to our transcriptomic analysis. (B) Transcriptional changes of the genes at 1, 6, and 24 h of UV-B radiation. Genes no.1-17, COO91_00791–00773 (excluding COO91_00785 and COO91_00789); genes no. 18-19, COO91_00772–00771; genes no. 20-24, COO91_05871–05866 (excluding COO91_05870).

Figure 5

The “C→C” conversion pathway of scytonemin biosynthesis. The direct precursors for scytonemin synthesis are Trp and p-Hpp, and the substrates for these two metabolites are PEP and E4P. Metabolite abbreviations: RuBP, ribulose-1,5-bisphosphat; Ru5P, ribulose-5-phosphate; Xu5P, xylulose-5-phosphate; GAP, glyceraldehyde-3-phosphate; 6PGN, 6-phosphate gluconate; G6P, glucose-6-phosphate; E4P, erythrose-4-phosphate. 3PGA, 3-phosphoglycerat; PEP, phosphoenolpyruvate; PYR, pyruvate; ACoA, acetyl-CoA; OAA, oxaloacetate; CA, Citrate; iosCA, iso-citrate; 2-OG, alpha-ketoglutarate; MA, malate; FA, fumarate; SuA, succinyl CoA. DAHP, 3-deoxy-D-arabino-heptulosonate-7-phosphate; DHQ, 3-dehydroquinate; DHS, 3-dehydroshikimate; SA, shikimate; S3P, SA-3-phosphate; EPSP, 5-enolpyruvyl-shikimate 3-phosphate; CHA, chorismate; 4-DG, 4-deoxygadusol; MAAs, mycosporine-like amino acids; Anth, anthranilate; Trp, tryptophan; Prep, prephenate; p-HPP, p-hydroxyphenylpyruvate; Phe, phenylalanine; Tyr, tyrosine; PheP, phenylpyruvate; Arog, arogenate. Indol-3-P, indole-3 pyruvate; Comp 1, intermediate compound 1; Comp 2, intermediate compound 2. Enzyme abbreviations: RuBisCO, ribulose-1,5-bisphosphate carboxylase/oxygenase; PEPc, PEP carboxylase.