The protein O 6-alkylguanine-DNA alkyltransferase(alkyltransferase) is involved 
in the repair of O 6-alkylguanine and O 4-alkylthymine in DNA and plays an 
important role in most organisms in attenuating the cytotoxic and mutagenic 
effects of certain classes of alkylating agents. A genomic clone encompassing 
the Drosophila melanogaster alkyltransferase gene ( DmAGT ) was identified on 
the basis of sequence homology with corresponding genes in Saccharomyces 
cerevisiae and man. The DmAGT gene is located at position 84A on the third 
chromosome. The nucleotide sequence of DmAGT cDNA revealed an open reading frame 
encoding 194 amino acids. The MNNG-hypersensitive phenotype of 
alkyltransferase-deficient bacteria was rescued by expression of the DmAGT cDNA. 
Furthermore, alkyltransferase activity was identified in crude extracts of 
Escherichia coli harbouring DmAGT cDNA and this activity was inhibited by 
preincubation of the extract with an oligonucleotide containing a single 
O6-methylguanine lesion. Similar to E.coli Ogt and yeast alkyltransferase but in 
contrast to the human alkyltransferase, the Drosophila alkyltransferase is 
resistant to inactivation by O 6-benzylguanine. In an E.coli lac Z reversion 
assay, expression of DmAGT efficiently suppressed MNNG-induced G:C-->A:T as well 
as A:T-->G:C transition mutations in vivo. These results demonstrate the 
presence of an alkyltransferase specific for the repair of O 6-methylguanine and 
O 4-methylthymine in Drosophila.