RNase HIII from Chlamydophila pneumoniae can efficiently cleave double‐stranded DNA carrying a chimeric ribonucleotide in the presence of manganese

Summary Two ribonuclease Hs (RNase Hs) have been found in Chlamydophila pneumoniae , CpRNase HII and CpRNase HIII. This work is the first report that CpRNase HIII can efficiently cleave DNA‐rN 1 ‐DNA/DNA (rN 1 , monoribonucleotide) in vitro in the presence of Mn 2+ , whereas the enzymatic activity of CpRNase HII on the same substrate was inhibited by Mn 2+ and dependent on Mg 2+ . However, the ability of both CpRNase Hs to cleave other alternative substrates (RNA/DNA hybrids and Okazaki‐like substrates), was insensitive to the divalent ions changes, suggesting that high concentrations of Mn 2+ specifically repressed the ability of CpRNase HII to cleave DNA‐rN 1 ‐DNA/DNA but activated this function in CpRNase HIII. Further in vivo experiments showed that the CpRNase HII complementation of Escherichia coli rnh ‐ mutations in an Mg 2+ environment was suppressed by Mn 2+ . In contrast, Mn 2+ was indispensable for CpRNase HIII to complement the same mutations. Further, the cell growth inhibition and the genomic DNA sensitivity to alkali in the bacterial strain lacking RNase HII activity could be relieved by functional CpRNase HII or HIII with its compatible ion. Therefore, CpRNase HIII can execute cleavage activity on DNA‐rN 1 ‐DNA/DNA under a Mn 2+ ‐rich environment and may function as a substitute for CpRNase HII under special physiological states.