Schizosaccharomyces pombe has two paralogues of 3-methyladenine DNA glycosylase, 
Mag1p and Mag2p, which share homology with Escherichia coli AlkA. To clarify the 
function of these redundant enzymes in base excision repair (BER) of alkylation 
damage, we performed several genetic analyses. The mag1 and mag2 single mutants 
as well as the double mutant showed no obvious methyl methanesulfonate (MMS) 
sensitivity. Deletion of mag1 or mag2 from an nth1 mutant resulted in tolerance 
to MMS damage, indicating that both enzymes generate AP sites in vivo by removal 
of methylated bases. A rad16 mutant that is deficient in nucleotide excision 
repair (NER) exhibited moderate MMS sensitivity. Deletion of mag1 from the rad16 
mutant greatly enhanced MMS sensitivity, and the mag2 deletion also weakened the 
resistance to MMS of the rad16 mutant. A mag1/mag2/rad16 triple mutant was most 
sensitive to MMS. These results suggest that the NER pathway obscures the mag1 
and mag2 functions in MMS resistance and that both paralogues initiate the BER 
pathway of MMS-induced DNA damage at the same level in NER-deficient cells or 
that Mag2p tends to make a little lower contribution than Mag1p. Mag1p and Mag2p 
functioned additively in vivo. Expression of mag1 and mag2 in the triple mutant 
confirmed the contribution of Mag1p and Mag2p to BER of MMS resistance.