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). Ribonuclease HI (RNase HI) is an endonuclease that cleaves the RNA strand of an RNA/DNA hybrid in a sequence non-specific manner. RNase H is widely present in various organisms, including bacteria, archaea and eukaryotes. RNase HI has also been observed as an adjunct domain to the reverse transcriptase gene in retroviruses, long-term repeat (LTR)-bearing retrotransposons and non-LTR retrotransposons. RNase HI in LTR retrotransposons perform degradation of the original RNA template, generation of a polypurine tract (the primer for plus-strand DNA synthesis), and final removal of RNA primers from newly synthesized minus and plus strands. The catalytic residues for RNase H enzymatic activity, three aspartatic acids and one glutamic acid residue (DEDD), are unvaried across all RNase H domains. The position of the RNase domain of non-LTR and LTR transposons is at the carboxyl terminal of the reverse transcriptase (RT) domain and their RNase domains group together, indicating a common evolutionary origin. Many non-LTR transposons have lost the RNase domain because their activity is at the nucleus and cellular RNase may suffice; however LTR retrotransposons always encode their own RNase domain because it requires RNase activity in RNA-protein particles in the cytoplasm. RNase H inhibitors have been explored as an anti-HIV drug target because RNase H inactivation inhibits reverse transcription.
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