Functional characterization of a ripening-specific L-methionine aminotransferase and its role in volatile C3-thioether esters biosynthesis in melon fruits.

C₃-thioether esters are among the most important contributors to melon aromas. We previously showed that L-methionine is a key precursor to many sulfur-containing and other volatiles in melon fruit. We also showed that L-methionine and its deaminated keto acid derivative, α-keto-γ-methylthio butyric acid, serve as precursors for C₃-thioethers in melon fruit. In this work, we have refined these findings and show that administering exogenous moderate L-methionine levels to melon fruit slices led to elevated levels of all detected sulfur-containing volatiles. In contrast, moderate levels of exogenous α-keto-γ-methylthio butyric acid specifically enhanced only the levels of C₃-thioethers and thioether esters. Cell-free extracts derived from ripe melon fruits were shown to possess L-methionine aminotransferase activity that preferentially accepted glyoxylate as a co-substrate for catalysis and other amine acceptors at lower rates. RNA-sequencing experiments indicated that the expression of CmMetAT, a gene annotated as a putative aminotransferase enzyme, is preferentially expressed in ripe fruit of PI 414723 and 'Dulce' melon cultivars, both in flesh and rind tissues. CmMetAT was ectopically expressed in E. coli and functionally characterized in vitro. CmMetAT encodes a bona fide L-methionine aminotransferase that accepts glyoxylate to generate α-keto-γ-methylthio butyric acid from L-methionine. Our findings indicate that CmMetAT mediates the formation of C₃-thioether volatiles in melon fruit. This work makes an additional step towards understanding the biosynthesis of the complex aroma constituents of melons and also projects to other fruits, especially those that accumulate sulfur-containing aroma volatiles.