The genetic and molecular basis for sunscreen biosynthesis in cyanobacteria

Abstract

Ultraviolet UV-A and UV-B radiation is harmful to living systems, causing damage to biological macromolecules. An important strategy for dealing with UV exposure is the biosynthesis of small-molecule sunscreens. Among such metabolites, the mycosporine and mycosporine-like amino acids (MAAs) are remarkable for their wide phylogenetic distribution and their unique chemical structures. Here, we report the identification of a MAA biosynthetic gene cluster in a cyanobacterium and the discovery of analogous pathways in other sequenced organisms. We have expressed the cluster in a heterologous bacterial host and characterized all four biosynthetic enzymes in vitro. In addition to clarifying the origin of the MAAs, these efforts have revealed two unprecedented enzymatic strategies for imine formation.

Figure 1. Mycosporines and MAAs

(A) Chemical structures of representative mycosporines and MAAs from fungi (mycosporine serinol) and cyanobacteria (mycosporine glycine, shinorine and porphyra-334). (B) Shinorine resonance tautomers. (C) UV-vis absorbance spectrum of shinorine.

Figure 2. Biosynthesis of shinorine

(A) The putative shinorine gene cluster from A. variabilis. (B) Biosynthetic pathway for the assembly of shinorine from sedoheptulose-7-phosphate. (C) HPLC traces of methanol extracts from E. coli induced with 500 μM IPTG at 15 °C (310 nm). Abbreviations: SH = shinorine, MG = mycosporine glycine, 4-DG = 4-deoxygadusol.

Figure 3. ¹⁸O labeling experiments

(A) Reaction of 4-deoxygadusol and ¹⁸O glycine with ATP-grasp homolog Ava_3856. (B) Reaction of mycosporine glycine and ¹⁸O serine with NRPS-like enzyme Ava_3855 (A = adenylation domain, T = thiolation domain, TE = thioesterase domain). (C) Extracted ion chromatograms from LC-MS analysis of Ava_3856 and Ava_3855 assays.