In response to heavy metal stress, plants and certain fungi, such as the fission yeast Schizosaccharomyces pombe, synthesize small metal‐binding peptides known as phytochelatins. We have identified a cadmium sensitive S. pombe mutant deficient in the accumulation of a sulfide‐containing phytochelatin‐cadmium complex, and have isolated the gene, designated hmt1, that complements this mutant. The deduced protein sequence of the hmt1 gene product shares sequence identity with the family of ABC (ATP‐binding cassette)‐type transport proteins which includes the mammalian P‐glycoproteins and CFTR, suggesting that the encoded product is an integral membrane protein. Analysis of fractionated fission yeast cell components indicates that the HMT1 polypeptide is associated with the vacuolar membrane. Additionally, fission yeast strains harboring an hmt1‐expressing multicopy plasmid exhibit enhanced metal tolerance along with a higher intracellular level of cadmium, implying a relationship between HMT1 mediated transport and compartmentalization of heavy metals. This suggests that tissue‐specific overproduction of a functional hmt1 product in transgenic plants might be a means to alter the tissue localization of these elements, such as for sequestering heavy metals away from consumable parts of crop plants.

Phytochelatins (PCs) are enzymically synthesized peptides produced in higher plants and some fungi upon exposure to heavy metals. We have examined PC production in the Se-tolerant wild mustard Brassica juncea and found that it produces two types of PC-Cd complexes with the same characteristics as those from fission yeast Schizosaccharomyces pombe, including a high molecular weight PC-Cd-sulfide form.Heavy metals, a group of elements including Cd, Cu, Zn, Bi, Ni, Hg, and Pb, are toxic to all organisms at varying concentrations. In response to those elements, animal cells and some fungi, including Saccharomyces cerevisiae and Neurospora crassa, synthesize metal-chelating proteins called metallothioneins (11, 13). In contrast, the fission yeast Schizosaccharomyces pombe, algae, and higher plants synthesize enzymically produced peptides as a result of exposure to these metals. These metal-binding peptides have been called PC,2 cadystins, Cd-peptides, y(EC),-G, and y-glutamyl peptides; they will be referred to here, as in recent reviews (24,28,30), as PCs. PCs were first discovered in the fission yeast as components of metal-containing complexes from Cd-induced cells (19,20); it was demonstrated that previous reports of metallothionein-like proteins from plants could be attributed to PCs (9) and that PCs were produced by all higher plants tested (7). PCs have also been shown to exist in the algae Chlorella fusca (6) and the yeast Candida glabrata, which produces both PCs and a metallothionein (17,18). PC synthesis is induced by a variety of metals, including

No abstract

Phytochelatins (PCs) are metal-chelating peptides produced in plants and some fungi in response to heavy metal exposure. A Cd-sensitive mutant of the fission yeast Schizosaccharomyces pombe, defective in production of a PC-Cd-sulfide complex essential for metal tolerance, was found to harbor mutations in specific genes of the purine biosynthetic pathway. Genetic analysis of the link between metal complex accumulation and purine biosynthesis enzymes revealed that genetic lesions blocking two segments of the pathway, before and after the IMP branchpoint, are required to produce the Cd-sensitive phenotype. The biochemical functions of these two segments of the pathway are similar, and a model based on the alternate use of a sulfur analog substrate is presented. The novel participation of purine biosynthesis enzymes in the conversion of the PC-Cd complex to the PC-Cd-sulfide complex in the fission yeast raises an intriguing possibility that these same enzymes might have a role in sulfur metabolism in the fission yeast S. pombe, and perhaps in other biological systems.

Phytochelatins (PCs) are metal-chelating peptides produced in plants and some fungi in response to heavy metal exposure. A Cd-sensitive mutant of the fission yeast Schizosaccharomyces pombe, defective in production of a PC-Cd-sulfide complex essential for metal tolerance, was found to harbor mutations in specific genes of the purine biosynthetic pathway. Genetic analysis of the link between metal complex accumulation and purine biosynthesis enzymes revealed that genetic lesions blocking two segments of the pathway, before and after the IMP branchpoint, are required to produce the Cd-sensitive phenotype. The biochemical functions of these two segments of the pathway are similar, and a model based on the alternate use of a sulfur analog substrate is presented. The novel participation of purine biosynthesis enzymes in the conversion of the PC-Cd complex to the PC-Cd-sulfide complex in the fission yeast raises an intriguing possibility that these same enzymes might have a role in sulfur metabolism in the fission yeast S. pombe, and perhaps in other biological systems.