A uracil-DNA glycosylase gene was cloned from Chlamydia pneumoniae AR39 and expressed in E coli strains BL21 (DE3) and BL21 (DE3) pLysS. After purification by Ni-NTA His Bind Resin and DEAE Sepharose Fast Flow column chromatography, recombinant CpUDG with a specific activity of 1,000,000 U/mg was obtained. The enzymatic activity of the purified CpUDG protein was further characterized using oligodeoxyribonucleotides carrying uracil bases as substrates. The base opposite to uracil in double strand DNAs affected uracil removal efficiencies in the order: U/- > U/T > U/C > U/G > U/A. Free uracil and abasic sites (AP site) could inhibit the reaction. The optimal temperature and pH for uracil removal by CpUDG were 37degreesC and pH 8.0, respectively. Site-directed mutagenesis studies indicated that amino acids D77, H200, and A205 were important for the catalytic activity of CpUDG. Together, these data suggest that CpUDG is a member of the UDG family-I protein. This is the first report on cloning, expression, and characterization of Chlamydia uracil-DNA glycosylase. (C) 2004 Elsevier Inc. All rights reserved.