A novel DNA damage recognition protein in Schizosaccharomyces pombe

Toxic and mutagenic O-6-alkylguanine adducts in DNA are repaired by O-6-alkylguanine-DNA alkyltransferases (MGMT) by transfer of the alkyl group to a cysteine residue in the active site. Comparisons in silico of prokaryotes and lower eukaryotes reveal the presence of a group of proteins [alkyltransferase-like (ATL) proteins] showing amino acid sequence similarity to MGMT, but where the cysteine at the putative active site is replaced by tryptophan. To examine whether ATL proteins play a role in the biological effects of alkylating agents, we inactivated the gene, referred to as atl1(+), in Schizosaccharomyces pombe, an organism that does not possess a functional MGMT homologue. The mutants are substantially more susceptible to the toxic effects of the methylating agents, N-methyl-N-nitrosourea, N-methyl-N'nitro-N-nitrosoguanidine and methyl methanesulfonate and longer chain alkylating agents including N-ethyl-N-nitrosourea, ethyl methanesulfonate, N-propyl-N-nitrosourea and N-butyl-N-nitrosourea. Purified Atl1 protein does not transfer methyl groups from O-6-methylguanine in [H-3]-methylated DNA but reversibly inhibits methyl transfer by human MGMT. Atl1 binds to short single-stranded oligonucleotides containing O-6-methyl, -benzyl, -4-bromothenyl or -hydroxyethyl-guanine but does not remove the alkyl group or base and does not cleave the oligonucleotide in the region of the lesion. This suggests that Atl1 acts by binding to O-6-alkylguanine lesions and signalling them for processing by other DNA repair pathways. This is the first report describing an activity that protects S.pombe against the toxic effects of O-6-alkylguanine adducts and the biological function of a family of proteins that is widely found in prokaryotes and lower eukaryotes.