Endonuclease that specifically degrades the RNA of RNA-DNA hybrids. RNase H participates in DNA replication; it helps to specify the origin of genomic replication by suppressing initiation at origins other than the oriC locus; along with the 5'-3' exonuclease of pol1, it removes RNA primers from the Okazaki fragments of lagging strand synthesis; and it defines the origin of replication for ColE1-type plasmids by specific cleavage of an RNA preprimer. Involved in production of retron derived msDNA (a branched RNA linked by a 2',5'-phosphodiester bond to a single-stranded DNA). Strain K12 / MG1655 does not have retrons, however they can be expressed in this strain which requires this enzyme (tested with retron Ec86) (PubMed:7568472). Endonucleolytic cleavage to 5'-phosphomonoester. Binds 1 Mg(2+) ion per subunit in the absence of substrate. Requires millimolar levels of Mg(2+) for maximal activity. Has low activity at micromolar concentrations of Mn(2+) and is inhibited at higher Mn(2+) levels. Can bind a second metal ion at a regulatory site, or after substrate binding. Monomer. In a strain that encodes a retron, the msDNA is abnormally sized. Belongs to the RNase H family.