Fig. 1

Direct- and trans-sulfurylation of methionine biosynthesis in bacteria. The first step in methionine biosynthesis involves the activation of homoserine through an acylation step. Two enzymes encoded by metAs and metXa genes [31] activate homoserine. The enzyme homoserine succinyl transferase (HST, MetAs) converts homoserine and succinyl-CoA into O-succinyl-L-homoserine (OSH). The enzyme homoserine acetyl transferase (HAT, MetXa) converts homoserine and acetyl-CoA into O-acetyl-L-homoserine (OAH). In the trans-sulfurylation pathway, cysteine and O-succinyl-L-homoserine (OSH) are converted into cystathionine by cystathionine-γ-synthase (CgS, MetB). Cystathionine is converted into homocysteine by cystathionine-β-lyase CbL (MetC). In the direct-sulfurylation pathway, OAH is converted into homocysteine by O-acetylhomoserine sulfhydrylase (OAHS, MetY). Metabolites in the pathways are boxed. MetJ and McbR are master negative regulators in E. coli and C. glutamicum, respectively. Additional abbreviations: MetF − 5,10-methylenetetrahydrofolate reductase, MetK - S-adenosylmethionine synthase, MetE - Cobalamin-independent methionine synthase, MetH - Methionine synthase, YjeH - L-methionine exporter, SAM - S-adenosyl-methionine