The metBL operon in E. coli contains two genes that encode proteins involved in the biosynthesis of methionine. The operon has known repressors, as well as a known activator. The operon can be activated by the binding of the activated transcriptional regulatory protein PhoP-phosphate complex to the PhoP binding site upstream of the promoter. The PhoP-phosphate complex is formed in a phosphorylation reaction where the sensor protein PhoQ donates its phosphate group to PhoP. The binding of the PhoP complex helps with recruitment of RNA polymerase, initiating transcription of the operon. The repressor of the operon is the Met repressor MetJ, which binds upstream of both the promoter and PhoP binding site. It can bind to multiple sites in this region, and repressor proteins interact with each other. When this interaction occurs, the Met repressor proteins form and bind to the operator of the operon, preventing RNA polymerase from binding and preventing transcription. The first gene in the operator, metB, encodes cystathionine gamma-synthase, an enzyme that catalyzes the formation of L,L-cystathionine and succinate from L-cysteine and O-succinyl-L-homomserine, as part of the de novo biosynthesis of L-methionine pathway. The second and final gene, metL, encodes bifunctional aspartokinase/homoserine dehydrogenase 2, four molecules of which form a complex with four molecules of bifunctional aspartokinase/homoserine dehydrogenase 1. This complex catalyzes the formation of L-aspartate 4-semialdehyde from L-homoserine, using NADP as a cofactor. This reaction is used as a part of the L-methionine biosynthesis pathway, as well as L-lysine and L-threonine biosynthesis pathways.

Operon: Methionine Biosynthesis References