A set of novel heavy-metal complexing peptides was isolated from plant cell suspension cultures; the structure of the peptides was established as (gamma-glutamic acid-cysteine)n-glycine (n = 3 to 7). These peptides appear upon induction of plant cells with heavy metals and represent the principal metal-binding activities in the cells. The name phytochelatin is proposed for this new class of natural products.

An enzyme has been discovered and characterized from Silene cucubalus cell suspension cultures that catalyzes the transfer of the y-glutamylcysteine dipeptide moiety of glutathione to an acceptor glutathione molecule or a growing chain of [Glu(-Cys)],-Gly oligomers, thus synthesizing phytochelatins, the metal-binding peptides of higher plants and select fungi. The enzyme was named y-glutamylcysteine dipeptidyl transpeptidase and given the trivial name phytochelatin synthase. The primary reaction catalyzed is [Glu(-Cys)]-Gly + [Glu(-Cys)],,-Gly -* [Glu(-Cys)],k+-Gly + Gly. The enzyme is isoelectric near pH 4.8 and has temperature and pH optima at 35'C and 7.9, respectively. Phytochelatin synthase is constitutively present in cell cultures of various plant species and its formation is not noticeably induced by heavy metal ions in the growth medium. The enzyme (Mr 95,000) seems to be composed of four subunits, the dimer (Mr 50,000) being also catalytically active. Cd2+ is by far the best metal activator of the enzyme followed by Ag+, Bi3+, Pb2+, Zn2+, Cu2+, Hg2+, and Au+. The Km for glutathione is 6.7 mM. The enzyme activity seems to be self-regulated in that the product of the reaction (the phytochelatins) chelates the enzyme-activating metal, thus terminating the enzyme reaction. The molar ratio of the y-glutamylcysteine dipeptide in phytochelatin to Cd2+ in the newly formed complex was 2:1.

Phytochelatins are a class of heavy-metalbinding peptides previously isolated from cell suspension cultures of several dicotyledonous and monocotyledonous plants. These peptides consist of repetitive y-glutamylcysteine units with a carboxyl-terminal glycine and range from 5 to 17 amino acids in length. In the present paper we show that all plants tested synthesized phytochelatins upon exposure to heavy metal ions. No evidence for the occurrence of metallothionein-like proteins was found. All data so far obtained indicate that phytochelatins are involved in detoxification and homeostasis of heavy metals in plants and thus serve functions analogous to those of metallothioneins in animals and some fungi. Phytochelatins are induced by a wide range of metal anions and cations. Phytochelatin synthesis in suspension cultures was inhibited by buthionine sulfoximine, a specific inhibitor of y-glutamylcysteine synthetase (EC 6.3.2.2). This finding and kinetic studies of phytochelatin induction point to a synthesis from glutathione or its precursor, r-glutamylcys-

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Transcription from one of the two initiation sites, P1 and P2, of the dual human MYC promoter seems to be essential in all proliferating cells. To identify proteins and target structures for MYC regulation, a DNA region was analyzed that is critical for P2 promoter activity. Here, we show that a nuclear factor binds to a DNA element within P2, which is conserved perfectly between mouse and man and displays a striking homology to the Ela-inducible E2 promoter of adenovirus type 5 (AdS}. We demonstrate that the same transcription factor, defined recently as E2F, which plays an essential role in the activation of adenovirus early promoters and enhancers, also interacts as a dominant nuclear factor with the MYC promoter. The presence of an intact E2F binding site is required for basic expression and for trans-activation of the P2 promoter by Ela proteins. The human MYC promoter is the first cellular target described for E2F. The results suggest that expression of MYC might be regulated via modulation of E2F by cellular 'Ela-like' factors.