Role of homocysteine in metR-mediated activation of the metE and metH genes in Salmonella typhimurium and Escherichia coli.

The metR-mediated activation of the Salmonella typhimurium metE and metH genes was shown to be modulated by homocysteine, an intermediate in the methionine biosynthetic pathway. Homocysteine stimulates expression of a metE-lacZ gene fusion four- to fivefold by increasing transcription from the metE promoter. In contrast, homocysteine plays an inhibitory role in the metR-mediated activation of the metH gene, decreasing expression of a metH-lacZ gene fusion threefold.