Electron microscopy of gene regulation: the L-arabinose operon

Unlike normal cells, malignant rat and two simian virus 40-transformed human cell lines can neither grow nor survive in B12- and folate-supplemented media in which methionine is replaced by homocysteine. Yet three lines of evidence indicate that the malignant and transformed cells synthesize large amounts of methionine endogenously through the reaction catalyzed by 5-methyltetrahydropterolyl-L-glutamate: L-homocysteine S-methyltransferase (EC 2.1.1.13). (1) The activities of this methyltransferase were comparable in extracts of malignant and normal cells. (2) The uptake of radioactive label from [5-14C]methyltetrahydropteroyl-L-glutamic acid (5-Me-H4PteGlu) was at least as great in the malignant cells as in the normals and was nearly totally dependent on the addition of homocysteine, the methyl acceptor; furthermore, 59-84% of the label incorporated by cells was recovered as methionine.