A role for HEM2 in cadmium tolerance.
A Candida glabrata cadmium-sensitive mutant partially defective in glutathione production and exhibiting a complete absence of phytochelatins was used to clone a gene required for Cd tolerance. Transformation of the Cd-sensitive mutant with a genomic library from the wild-type C. glabrata led to the cloning of a gene that restored Cd tolerance and formation of Cd-glutathione and Cd-phytochelatin complexes. The cloned gene showed high levels of nucleic acid and protein sequence homology to the HEM2 genes, encoding porphobilinogen synthases, from several sources. It was shown that the C, glabrata Cd-sensitive mutant indeed exhibited a significant reduction in porphobilinogen synthase levels. The cloned C. glabrata gene complemented a hem2 mutant of Saccharomyces cerevisiae and restored porphobilinogen synthase activity in the mutant. The Cd sensitive mutant predictably showed decreased levels of sulfite reductase that requires siroheme, a metabolite produced in the heme biosynthetic pathway. The addition of cysteine, but not methionine, increased glutathione levels and Cd tolerance of both the wild-type and the mutant strain. However, addition of hemin chloride and methionine together restored Cd tolerance indicating that heme was required for transsulfuration of homocysteine to cysteine.