cDNA cloning of the rat O6-methylguanine-DNA-methyltransferase.

A cDNA expression library was constructed from a rat hepatoma cell line ( H4 cells ) and introduced into an Escherichia coli strain ( BK2110 ) deficient in the repair of O6-methylguanine residues. Following three exposures to N-methyl-N'-nitro-N-nitrosoguanidine, a resistant colony harboring a plasmid named RMGMT was isolated. Extracts of BK2210 cells hosting the RMGMT plasmid expressed a O6-methylguanine-DNA-methyltransferase (transferase) activity and this protein had the same molecular weight as the transferase from H4 cells. The cDNA sequence of 763 bp contains an open reading frame of 630 bp encoding a protein of 209 amino acids with a calculated molecular weight of 22.2 kd. The rat protein shows 68% homology with the human transferase.