A read-ahead function in archaeal DNA polymerases detects promutagenic template-strand uracil

Deamination of cytosine to uracil is the most common promutagenic change in DNA, and it is greatly increased at the elevated growth temperatures of hy perthermophilic archaea, If not repaired to cytosine prior to replication, uracil in a template strand directs incorporation of adenine, generating a G.C --> A.U transition mutation in half the progeny. Surprisingly, genomic analysis of archaea has so far failed to reveal any homologues of either of the known families of uracil-DNA glycosylases responsible for initiating t he base-excision repair of uracil in DNA, which is otherwise universal. Her e we show that DNA polymerases from several hyperthermophilic archaea (incl uding Vent and Pfu) specifically recognize the presence of uracil in a temp late strand and stall DNA synthesis before mutagenic misincorporation of ad enine. A specific template-checking function in a DNA polymerase has not be en observed previously, and it may represent the first step in a pathway fo r the repair of cytosine deamination in archaea.