Decoding Specificity of Cyanobacterial MysDs in Mycosporine-Like Amino Acid Biosynthesis through Heterologous Expression in Saccharomyces cerevisiae

Mycosporine-like amino acids (MAAs) are potent natural UV-protectants, but their industrial production is hindered by efficiency and sustainability issues of large-scale extraction of their native hosts. Heterologous expression of MAA biosynthesis pathway genes in chassis organisms provides a promising alternative route, though the substrate promiscuity of the ATP-grasp ligase MysD complicates the biosynthesis of specific MAAs. In this study, we developed a Saccharomyces cerevisiae strain with enhanced capacity of producing mycosporine-glycine (MG), through genomic expression of biosynthesis pathway genes and knockout of competing pathway genes. This strain serves as an efficient MysD expression platform, which converts MG into shinorine and porphyra-334. Through structural modelling, site-directed mutagenesis and mutant characterization, we identified two residues on the omega-loop of MysD involved in determining product specificity. We further characterized the product specificity of 20 MysDs from diverse cyanobacterial lineages and confirmed the residue pattern-product specificity correlation. Our findings provide guidance for screening, selecting, and designing novel MysDs for industrial-scale MAA production through heterologous expression.