Mimosine attenuates serine hydroxymethyltransferase transcription by chelating zinc. Implications for inhibition of DNA replication.

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 mimosine-induced 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.