Discovery of a novel methionine biosynthetic route <i>via O</i> -phospho- <scp>l</scp> -homoserine

Methionine (Met), a sulfur-containing amino acid, is essential for the underlying biological processes in living organisms. In addition to its importance as a starting building block for peptide chain elongation in protein biosynthesis, Met is a direct precursor of <i>S</i>-adenosyl-l-methionine, an indispensable methyl donor molecule in primary and secondary metabolism. <i>Streptomyces</i> bacteria are well known to produce diverse secondary metabolites, but many strains lack canonical Met pathway genes for l-homocysteine, a direct precursor of Met in bacteria, plants, and archaea. Here, we report the identification of a novel gene (<i>metM</i>) responsible for the Met biosynthesis in <i>Streptomyces</i> strains and demonstrate the catalytic function of the gene product, MetM. We further identified the <i>metO</i> gene, a downstream gene of <i>metM</i>, and showed that it encodes a sulfur-carrier protein (SCP). In <i>in vitro</i> analysis, MetO was found to play an important role in a sulfur donor by forming a thiocarboxylated SCP. Together with MetO (thiocarboxylate), MetM directly converted <i>O</i>-phospho-l-homoserine to l-homocysteine. <i>O</i>-Phospho-l-homoserine is also known as an intermediate for threonine biosynthesis in bacteria and plants, and MetM shares sequence homology with threonine synthase. Our findings thus revealed that MetM seizes <i>O</i>-phospho-l-homoserine from the threonine biosynthetic pathway and uses it as an intermediate of the Met biosynthesis to generate the sulfur-containing amino acid. Importantly, this MetM/MetO pathway is highly conserved in <i>Streptomyces</i> bacteria and distributed in other bacteria and archaea.IMPORTANCEMethionine (Met) is a sulfur-containing proteinogenic amino acid. Moreover, Met is a direct precursor of <i>S</i>-adenosyl-l-methionine, an indispensable molecule for expanding the structural diversity of natural products. Because Met and its derivatives benefit humans, the knowledge of Met biosynthesis is important as a basis for improving their fermentation. <i>Streptomyces</i> bacteria are well known to produce diverse and valuable natural products, but many strains lack canonical Met pathway genes. Here, we identified a novel l-homocysteine synthase (MetM) in <i>Streptomyces</i> and demonstrated that it converts <i>O</i>-phospho-L-homoserine to l-homocysteine using a thiocarboxylated sulfur-carrier protein as a sulfur donor. Since the <i>metM</i> is distributed in other bacteria and archaea, our pioneering study contributes to understanding Met biosynthesis in these organisms.