The E. coli ogt O6-alkylguanine-DNA alkyltransferase has two cysteine residues 
positioned identically with respect to cysteines in the E. coli ada 
O6-alkylguanine-DNA alkyltransferase. In order to assess their function, these 
residues were each substituted by a glycine to generate altered forms of the ogt 
protein. Mutagenesis of cysteine-139, located within a 'PCHRV' region of 
homology, eliminated functional activity confirming that this residue is the 
methyl-accepting cysteine in the active site of the protein. Substitution of 
cysteine 102 within the sequence 'LRTIPCG' had little effect on the ogt protein 
activity demonstrating that this cysteine is not directly involved with the 
transfer of O6-methylguanine adducts.