Class I Methyltransferase VioH Catalyzes Unusual S-Adenosyl-L-methionine Cyclization Leading to 4-Methylazetidinecarboxylic Acid Formation during Vioprolide Biosynthesis

S-Adenosyl-L-methionine (SAM)-dependent methyltransferases are intensely studied because they play important roles in the methylation of biomolecules in all domains of life. In this study, we describe that the methyltransferase VioH from Cysotobacter violaceus catalyzes a so far unknown cyclization of SAM to azetidine-2-carboxylic acid (AZE), which is proposed to be the precursor of the unusual 4-methylazetidinecarboxylic acid (MAZ) moiety of vioprolides. In vitro biochemical investigations reveal that SAM is converted to AZE in the presence of VioH while MAZ is generated by coexpression of VioH and the radical SAM enzyme VioG in Myxococcus xanthus or by combination of VioH and the cell lysate of M. xanthus expressing VioG. Thus, our findings unveil a novel function of SAM-dependent methyltransferases and shed light on the biosynthetic mechanism of MAZ formation.