Nucleotide sequence of the Streptococcus pneumoniae ung gene encoding uracil-DNA glycosylase.

Uracil-DNA glycosylase, the enzyme responsible for the removal of uracil from DNA (1), is directly involved in mutation avoidance (2). Indeed, it is likely to prevent transition mutations by removing uracil that results from deamination of cytosine. It has been proposed that the removal of misincorporated uracil by uracil-DNA glycosylase also plays an indirect role in correction of replication errors in nascent strands (3): single stranded gaps resulting from the removal of uracil might target the generalized mismatch repair system of Streptococcus pneumoniae (4) to nascent strands. With the aim of investigating the role of uracil-DNA glycosylase, we generated a ung~ mutant and characterized the ung gene of 5. pneumoniae. We report its nucleotide sequence here (fig. 1). The enzyme appears highly conserved from human to Gram" (Escherichia coif) (5) and to Gram (5. pneumoniae) bacteria (fig. 2). Investigation of mutation rates offers no support to the hypothesis of a role of the enzyme in targeting generalized mismatch repair (V.M., J.-C. Dev&Ijian, I.R., G. Alloing and J.-P.C, in preparation). Nevertheless, the ubiquity and conservation of uracil-DNA glycosylases are witnesses of a biologically significant direct role in mutation avoidance.