UGT74S1 Is Unique in Controlling Secoisolariciresinol Diglucoside (SDG) Formation in Flax

Flax secoisolariciresinol diglucoside (SDG) lignan is a natural glycosylated diphenolic phytoestrogen for which a positive role in human metabolic diseases is emerging. As part of the Genome Canada flax TUFGEN genomics project, UDP-glucosyl transferase UGT74S1 was identified and characterized as an enzyme sequentially glucosylating secoisolariciresinol (SECO) into secoisolariciresinol monoglucoside (SMG) and SDG when expressed in yeast. However, it remained unknown whether UGT74S1was the only UGT achieving these glucosylation steps in flax. In this study, UGT74S1gene duplication in the genome was investigated. Its closely related genes among the 299 UGTs present in the flax draft genome were identified and their enzyme activities toward SECO, UDP-glucose and UDP-galactose were determined in vitro. No evidence for UGT74S1 gene duplication was observed. Instead, it formed a small gene family with six other UGTs including LuS10008742 (UGT74S2), LuS10006721 (UGT74S5), LuS10006351 (UGT74S25), LuS10024486 (UGT74S3), LuS10014148 (UGT74S4), and LuS10006353 (UGT74S3), the latter two being closely related to UGT74S1, albeit located each on different flax chromosomes. LuS10014148 (UGT74S4) and LuS10006353 (UGT74S3) proteins showed seven times lower glucosylation activity towards SECO and failed to produce SDG compared UGT74S1. All three enzymes formed very low amounts of secoisolariciresinol monogalactoside (SMGal) and no secoisolariciresinol digalactoside (SDgal).  The study confirmed our previous findings in flax and suggests that UGT74S1 is unique in controlling SDG formation in flax although a small UGT gene family may also be contributory in supplying the precursor SMG for UGT74S1.