Organization and expression of the human gene for O⁶-methylguanine-DNA methyltransferase

O⁶-Methylguanine-DNA methyltransferase plays an important role in cellular defence against mutagens and carcinogens with alkylating activity. Certain tumor-derived cell lines, termed Mer^-, are defective in the enzyme activity and have an increased sensitivity to alkylating agents. We cloned the genomic sequence coding for the human O⁶-methylguanine-DNA methyltransferase and elucidated the structure. The gene consisted of 5 exons and spanned more than 170 kb, while mRNA for the enzyme was 950 nucleotides long. No or only little mRNA for the enzyme was formed in Mer^- cells, though there was no gross difference in the coding and promoter regions of the gene between Mer⁺ and Mer^- cells. The putative promoter region, derived from Mer⁺ cells, was placed upstream of the chloramphenicol acetyltransferase reporter gene and the constructs were introduced into Mer⁺ and Mer^- cells. In Mer^- cells, a lowered level of transient expression of the gene was observed as compared with Mer⁺ cells, but this difference alone does not account for the in vivo difference of expression of the gene in the two types of cells; there might be difference in cis-acting elements. The DNA sequence in the 5′ upstream region of the gene was extremely GC-rich and there were no consensus sequences, such as the TATA and CAAT boxes. There were lower levels of methylation in the putative promoter of various Mer^- cells, as compared with findings in Mer⁺ cells. Methylation in this region may be involved in regulating expression of the gene.