Novel DNA repair alkyltransferase from Caenorhabditis elegans

O6-alkylguanine DNA-alkyltransferase (AGT) is a widely distributed DNA repair protein that protects living organisms from endogenous and exogenous alkylation damage to DNA at the O6-position of guanine. The search of the C. elegans genome database for an AGT protein revealed the presence of a protein (cAGT-2) with some similarity to known AGTs in addition to the easily recognized cAGT-1 protein. The predicted protein sequence of cAGT-2 contains the amino acid sequence -ProCysHisPro- at the presumed active site of the protein, whereas all other known AGTs have -ProCysHisArg-. A truncated version of the cAGT-2 protein was expressed in E. coli. This purified recombinant protein was able to repair O6-methylguanine and O4-methylthymine adducts in DNA in vitro and also reacted with the bulky benzyl adduct in O6-benzylguanine. This fragment of cAGT-2 (104 amino acids) is the smallest protein possessing AGT activity yet described. The full-length cAGT-2 protein (274 amino acids) totally lacks the N-terminal domain present in all other known AGTs but has a long C-terminal extension that has significant homology to histone 1C. Expression of cAGT-2 in an E. coli strain lacking endogenous AGT activity provided modest but statistically significant resistance to the toxicity of N-methyl-N'-nitro-N-nitrosoguanidine, confirming that cAGT-2 is an alkyltransferase.