Genome-wide transcriptome profiling and characterization of mannuronan C5-epimerases in Saccharina japonica

Abstract

Alginate is a linear polysaccharide comprised of 1,4-linked-β-D-mannuronic acid (M) and α-L-guluronic acid (G). As the major cell wall polysaccharide in brown algae, alginate is widely used in the food, pharmaceutical, and chemical industries. Mannuronan C5-epimerases (MC5Es) catalyze the specific conversion of M to its C-5 epimer G, playing a key role in the modification of alginate. In this study, 143 MC5E gene sequences from Saccharina japonica (SjMC5Es) were identified based on the genome and transcriptome data. Transcriptomics analysis was conducted, and the results showed that the SjMC5Es varied in their spatial and temporal expression patterns. Most SjMC5Es exhibited higher expression in the basal blades of the algae at earlier developmental stages than at later adult stages, and the transcription levels of the SjMC5Es showed a clear trend of gradual increase or decrease from the basal to the distal blades. In addition, the β-D-mannuronic acid/α-L-guluronic acid (M/G) ratio of alginate from all samples was measured, and the M/G ratios were associated with the transcription levels of the SjMC5E genes. Finally, the highly expressed SjMC5E2 was characterized, soluble SjMC5E2 was heterologously expressed, and its mannuronan C5-epimerase activity was confirmed by 1H-nuclear magnetic resonance analysis. Our results provide a global view of the SjMC5Es in S. japonica and lay a foundation for further applications of MC5Es in improving the physiochemical properties of alginate.