This family includes type 2 RNases H from several bacteria, such as Bacillus subtilis, which have two different RNases, HII and HIII. RNases HIII are distinguished by having a large (70-90 residues) N-terminal extension of unknown function. In addition, the active site of RNase HIII differs from that of other RNases H; replacing the fourth residue (aspartate) of the acidic "DEDD" motif with a glutamate. Most prokaryotic and eukaryotic genomes contain multiple RNase H genes; however, no prokaryotic genomes contain the combination of both RNase HI and HIII. This mutual exclusive gene inheritance might be the result of functional redundancy of RNase HI and HIII in prokaryotes. Ribonuclease (RNase) H is classified into two families, type I (prokaryotic RNase HI, eukaryotic RNase H1 and viral RNase H) and type II (prokaryotic RNase HII and HIII, archaeal RNase HII and eukaryotic RNase H2/HII). RNase H endonucleolytically hydrolyzes an RNA strand when it is annealed to a complementary DNA strand in the presence of divalent cations, in DNA replication or repair.
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