The Neurospora crassa chr-1 gene is up-regulated by chromate and its encoded CHR-1 protein causes chromate sensitivity and chromium accumulation

Abstract

The ChrA membrane protein belongs to the CHR superfamily of chromate ion transporters, which includes homologues from bacteria, archaea and eukaryotes. Bacterial ChrA homologues confer chromate resistance by exporting chromate ions from the cell's cytoplasm. The Neurospora crassa strain 74-A chr-1 gene encodes a putative CHR-1 protein of 507 amino acid residues, which belongs to the CHR superfamily. RT-PCR assays showed that expression of the chr-1 gene was up-regulated by chromate exposure of N. crassa cultures. Introduction in N. crassa of sense and antisense fragments of the chr-1 gene, as part of a silencing module within the pSilent-1 vector, produced transformants with a phenotype of resistance to chromate and diminished accumulation of chromium, as compared with the control strain containing only the vector. A chromate-resistance phenotype was also observed in N crassa strains deleted in the genomic chr-1 gene, thus confirming that the absence of CHR-1 protein confers chromate resistance to the fungus. The cDNA from N. crassa chr-1 gene (Ncchr-1) was cloned into the pYES2 vector under the control of a GAL promoter and the resulting recombinant plasmid was transferred to the yeast Saccharomyces cerevisiae. Galactose-induced S. cerevisiae transformants expressing Ncchr-1 were more sensitive to chromate and accumulated 2.5 times more chromium than the induced strain containing only the vector. Excess sulfate, a chromate analog, was unable to protect S. cerevisiae chr-1 transformants from chromate toxicity. These data indicate that the N. crassa CHR-1 protein functions as a transporter that takes up chromate; it also appears that this transport occurs in a sulfate-independent fashion. This is the first report assigning a role as a chromate transporter to a nonbacterial CHR protein.