Biological role of the major AP (abasic site) endonuclease of an archaeon from geothermal environments.

Archaea and bacteria in geothermal environments are predicted to suffer DNA depurination in vivo at high rates, which raises questions regarding the biological roles of their abasic-site-repair enzymes. Gene deletion and enzymatic assay demonstrated that the saci_0015 gene of Sulfolobus acidocaldarius encodes an AP endonuclease (Apn) accounting for as much as 95% of the assayable activity in cell extracts and is not essential for viability. To identify genetic functions of this enzyme, deletion (ΔApn) strains were examined with respect to growth, spontaneous mutation, transformation by ssDNA containing an abasic site, and conjugation. Relative to its isogenic control, the ΔApn strain did not exhibit any change in growth rate or final cell density, rate or spectrum of spontaneous mutation, transformation by DNA containing an abasic site, or efficiency of DNA transfer and recombination. The apparent lack of genetic impact of removing the major AP endonuclease was unexpected and indicated that abasic sites are rarely bypassed directly by DNA polymerases in S. acidocaldarius. AP endonuclease deficiency had no obvious effect on survival of S. acidocaldarius under several test conditions, but it accelerated the death of cells at 4º C under illumination. Our results suggest that the normal level of AP endonuclease in S. acidocaldarius is well above the minimum required for growth and cell division but not for recovery from prolonged exposure to certain low-temperature conditions. This situation illustrates a biological challenge that has not been emphasized in experimental studies of extremophiles, i.e., the problem of long-term survival under "non-extreme" conditions.