Eukaryotic RNase H is essential and is longer and more complex than their prokaryotic counterparts.
Ribonuclease H (RNase H) is classified into two families, type 1 (prokaryotic RNase HI, eukaryotic RNase H1 and viral RNase H) and type 2 (prokaryotic RNase HII and HIII, and eukaryotic RNase H2). RNase H is an endonuclease that cleaves the RNA strand of an RNA/DNA hybrid in a sequence non-specific manner. RNase H is involved in DNA replication, repair and transcription. One of the important functions of RNase H is to remove Okazaki fragments during DNA replication. RNase H is widely present in various organisms, including bacteria, archaea and eukaryote and most prokaryotic and eukaryotic genomes contain multiple RNase H genes. Despite the lack of amino acid sequence homology, type 1 and type 2 RNase H share a main-chain fold and steric configurations of the four acidic active-site (DEDD) residues and have the same catalytic mechanism and functions in cells. Eukaryotic RNase H is longer and more complex than in prokaryotes. Almost all eukaryotic RNase HI have highly conserved regions at their N-termini called hybrid binding domain (HBD). It is speculated that the HBD contributes to binding the RNA/DNA hybrid. Prokaryotes and some single-cell eukaryotes do not require RNase H for viability, but RNase H is essential in higher eukaryotes. RNase H knockout mice lack mitochondrial DNA replication and die as embryos.
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