Base excision repair of DNA alkylation damage is initiated by a methylpurine DNA 
glycosylase (MPG) function. Such enzymes have previously been characterized from 
bacteria and eukarya, but not from archaea. We identified activity for the 
release of methylated bases from DNA in cell-free extracts of Archaeoglobus 
fulgidus, an archaeon growing optimally at 83 degrees C. An open reading frame 
homologous to the alkA gene of Escherichia coli was overexpressed and identified 
as a gene encoding an MPG enzyme (M(r) = 34 251), hereafter designated afalkA. 
The purified AfalkA protein differs from E. coli AlkA by excising alkylated 
bases only, from DNA, in the following order of efficiency: 3-methyladenine 
(m(3)A) >> 3-methylguanine approximately 7-methyladenine >> 7-methylguanine. 
Although the rate of enzymatic release of m(3)A is highest in the temperature 
range of 65-75 degrees C, it is only reduced by 50% at 45 degrees C, a 
temperature that does not support growth of A. fulgidus. At temperatures above 
75 degrees C, nonenzymatic release of methylpurines predominates. The results 
suggest that the biological function of AfalkA is to excise m(3)A from DNA at 
suboptimal and maybe even mesophilic temperatures. This hypothesis is further 
supported by the observation that the afalkA gene function suppresses the 
alkylation sensitivity of the E. coli tag alkA double mutant. The amino acid 
sequence similarity and evolutionary relationship of AfalkA with other MPG 
enzymes from the three domains of life are described and discussed.