O6-Methylguanine-DNA methyltransferase (MGMT; DNA-O6-methylguanine:protein-L-cysteine S-methyltransferase, EC 2.1.1.63), a unique DNA repair protein present in most organisms, removes the carcinogenic and mutagenic adduct O6-alkylguanine from DNA by stoichiometrically accepting the alkyl group on a cysteine residue in a suicide reaction. The mammalian protein is highly regulated in both somatic and germ-line cells. In addition, the toxicity of certain alkylating drugs in tumor and normal cells is inversely related to the levels of this protein. The cDNA of the human gene, henceforth named MGMT, has been cloned in an expression vector on the basis of its rescue of a methyltransferase-deficient (ada-) Escherichia coli host. A 22-kDa active methyltransferase encoded entirely by the cDNA contains an amino acid sequence of 61 residues that bears 60-65% similarity with segments of E. coli methyltransferase (products of the ada and ogt genes), which encompass the alkyl-acceptor residues. The human cDNA has no sequence similarity with the ada and ogt genes, due in part to differences in codon usage, and shows no detectable homology with E. coli genomic DNA. However, it hybridizes with distinct restriction fragments of human, mouse, and rat DNAs. The lack of methyltransferase observed in many human cell lines is due to the absence of the MGMT gene or to lack of synthesis and/or stability of its 0.95-kilobase poly(A)+ RNA transcript.

The nucleotide sequence of the trout metallothionein A (MT-A) regulatory region has been determined by dideoxy-primer sequencing. The genomic DNA fragment containing the 5' untranslated region was isolated from an EMBL-3 library containing trout sperm DNA partially digested with BamHI. The library was screened by hybridization with a 30-mer oligonucleotide probe complementary to the cDNA sequence of the trout MT-A gene (1). Computer analysis of the sequence revealed two regions of 16 nt each (underlined and bold in the sequence figure) that were highly similar to the consensus mammalian metal regulatory region (MRE). The two MREs (MRE-a in the sense strand and MRE-b in the complementary strand) of the trout MT-A share 15 of 16 and 13 of 16 nucleotides with the two elements reported for the trout metallothionein B (MT-B) putative promoter region (2). In contrast to the GC-rich mammalian MT regulatory regions, the 500 bp of the 5'-flanking region of trout MT-A were found to be 63% AT-rich. REFERENCES 1.