L-Mimosine is a naturally occurring plant amino acid and iron chelator that arrests the cell cycle in the late G 1 phase, although its mechanism of action is not known. Some studies indicate that mimosine prevents the initiation of DNA replication, whereas other studies indicate that mimosine disrupts elongation of the replication fork by impairing deoxyribonucleotide synthesis by inhibiting the activity of the iron-dependent enzyme ribonucleotide reductase and the transcription of the cytoplasmic serine hydroxymethyltransferase gene (SHMT1). In this study, the mechanism for mimosineinduced inhibition of SHMT1 transcription was elucidated. A mimosine-responsive transcriptional element was localized within the first 50 base pairs of the human SHMT1 promoter by deletion analyses and gel mobility shift assays. The 50-base-pair sequence contains a consensus zinc-sensing metal regulatory element (MRE) at position ؊44 to ؊38, and mutation of the MRE attenuated mimosine-induced transcription repression. Mimosine treatment eliminated MRE-and Sp1-binding activity in nuclear extracts from MCF-7 cells but not in nuclear extracts from a mimosine-resistant cell line, MCF-7/2a. MCF-7 cells cultured in zinc-depleted medium for more than 16 days were viable and lacked cytoplasmic serine hydroxymethyltransferase protein, confirming that mimosine inhibits SHMT1 transcription by chelating zinc. The disruption of DNA-protein interactions by zinc chelation provides a general mechanism for the inhibitory effects of mimosine on nuclear processes, including replication and transcription. Furthermore, this study establishes that SHMT1 is a zinc-inducible gene, which provides the first mechanism for the regulation of folate-mediated one-carbon metabolism by zinc.Mimosine is an amino acid found in plants of the Mimosa and Leucaena genera and a tyrosine analog (1). It is a cell-specific inhibitor of DNA replication in vivo; sensitive cell lines include human MCF-7 (2), HL60 (3) and Chinese hamster ovary (4), whereas human neuroblastoma (2), K562 (3), and Xenopus and mouse embryonic cells are resistant (5, 6). Mimosine is an effective iron chelator and a structural analog of deferiprone, a compound that is used clinically to lower systemic iron in patients who suffer from iron overload (7, 8) (Fig. 1).Mimosine arrests cell cycle progression in late G 1 and although it elicits a plethora of biological effects, its mechanism of action has not been established. Mimosine has been proposed to arrest DNA synthesis at the replication fork by repressing deoxyribonucleotide synthesis (9 -12). Mimosine and deferoxamine inhibit the activity of the iron-dependent enzyme ribonucleotide reductase in vitro by chelating iron (9, 11, 12) and deplete cellular deoxyribonucleotide pools in some but not all studies (2, 12). Furthermore, mimosine and deferoxamine have been shown to inhibit the transcription of the cytoplasmic serine hydroxymethyltransferase gene (SHMT1) 1 in MCF-7 cells but not in mimosine-resistant SH-SY5Y neuroblastoma (2). The cytoplasmic ser...