Nuclease that specifically degrades the RNA of RNA-DNA hybrids; seems to act exonucleolytically on RNA/DNA hybrids. Endonucleolytically removes RNA primers from the Okazaki fragments of lagging strand synthesis on its own. Complements the temperature-sensitive phenotype of an E.coli double rnhA/rnhB (RNase H) disruption mutant. Endonucleolytic cleavage to 5'-phosphomonoester. Divalent cations; Mn(2+) and Mg(2+) are preferred over Co(2+) or Ni(2+). Divalent cations are required for folding and substrate-binding, while divalent cations and NaCl are required for maximal protein stability. Belongs to the RNase H family.