Mutation spectrum induced by singlet oxygen in Escherichia coli deficient in exonuclease III

The repair of singlet oxygen (O-1(2))-induced DNA lesions requires several enzymes of the nucleotide and base excision repair pathways, including exon uclease III and endonuclease IV that are known apurinic/apyrimidinic-endonu cleases in Escherichia coli, In order to better understand the relevance of exonuclease III on the repair of these lesions, we investigated the mutage nic events that result from the replication of a O-1(2)-damaged plasmid in an exonuclease-deficient host (xth). The mutation spectrum in the tRNA supF gene target indicated that the absence of exonuclease UT does not change t he types of mutations induced by O-1(2) (mostly of G:C --> T:A and G:C --> C:G transversions). However, the spectrum shows that the mutations are scat tered in the supF gene, which is significatively different from the one obt ained in wild-type bacteria. Thus, exonuclease III may act on the repair of O-1(2)-induced lesions altering the DNA repair sequence specificity.