The gcvTHP operon in E. coli contains three genes that encode proteins involved in the glycine cleavage system. This system consists of four proteins, three of which are encoded in this operon, the other being the lipoamide dehydrogenase Lpd protein encoded by the lpd gene. Activation of the system occurs when large concentration of glycine are found in the cell, forming carbon dioxide, ammonia, a reduced pyridine nucleotide that cam be reused, and 5,10-methylenetetrahydrofolate. The operon can be inactivated when the glycine cleavge system transcriptional activator (GcvA) binds to one of four binding sites upstream of the promoter. If glycine levels in the cell are low, GcvA binds successfully, preventing transcription of the operon. However, if there is a high concentration of glycine in the cell, the cleavage system will need to be activated, and glycine can bind to GcvA, preventing its binding upstream of the promoter, and allowing transcription of the operon. The operon can also be inhibited by the transcriptional repressor PurR. Hypoxanthine can bind to PurR, activating it and allowing it to bind to the promoter region of the operon, blocking RNA polymerase and preventing the transcription of the operon. The first gene in the operon, gcvT, encodes an aminomethyltransferase also known as the glycine cleavage system T-protein. This protein transfers the amino group from the intermediate produced by the H-protein, as well as the formation of 5,10-methylenetetrahydrofolate. The second gene, gcvH, encodes the glycine cleavage system H-protein. This protein transfers the methylamine group of glycine between the P and T-proteins of the cleavage system. The final gene in the operon, gcvP, encodes a decarboxylating glycine dehydrogenase, also known as the glycine cleavage system P-protein. This protein binds to and cleaves the amino group of glycine, transferring it to the complex with the H-protein and releasing carbon dioxide.

Operon: Glycine Cleavage System Inactivation References