RNases H are a family of endonucleases that hydrolyze RNA residues in various nucleic acids. These enzymes are present in all branches of life, and their counterpart domains are also found in reverse transcriptases (RTs) from retroviruses and retroelements. RNases H are divided into two main classes (RNases H1 and H2 or type 1 and type 2 enzymes) with common structural features of the catalytic domain but different range of substrates for enzymatic cleavage. Additionally, a third class is found in some Archaea and bacteria. Besides distinct cellular functions specific for each type of RNases H, this family of proteins is generally involved in the maintenance of genome stability with overlapping and cooperative role in removal of R-loops thus preventing their accumulation. Extensive biochemical and structural studies of RNases H provided not only a comprehensive and complete picture of their mechanism but also revealed key basic principles of nucleic acid recognition and processing. RNase H1 is present in prokaryotes and eukaryotes and cleaves RNA in RNA/DNA hybrids. Its main function is hybrid removal, notably in the context of R-loops. RNase H2, which is also present in all branches of life, can play a similar role but it also has a specialized function in the cleavage of single ribonucleotides embedded in the DNA. RNase H3 is present in Archaea and bacteria and is closely related to RNase H2 in sequence and structure but has RNase H1-like biochemical properties. This review summarizes the mechanisms of substrate recognition and enzymatic cleavage by different classes of RNases H with particular insights into structural features of nucleic acid binding, specificity towards RNA and/or DNA strands and catalysis.

中文翻译：

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RNases H：结构和机制。

RNase H是内切核酸酶家族，可水解各种核酸中的RNA残基。这些酶存在于生活的所有分支中，它们的对应结构域也存在于逆转录病毒和逆转录元件的逆转录酶（RTs）中。RNase H分为两大类（RNase H1和H2或1型和2型酶），它们具有催化结构域的共同结构特征，但用于酶促裂解的底物范围不同。此外，在一些古细菌和细菌中发现了第三类。除了对每种类型的RNases H具有特定的独特细胞功能外，该蛋白家族通常还参与基因组稳定性的维持，在去除R环时具有重叠和协同作用，从而阻止了它们的积累。RNases H的广泛生化和结构研究不仅提供了其机理的全面而完整的图片，而且揭示了核酸识别和加工的关键基本原理。RNase H1存在于原核生物和真核生物中，并切割RNA / DNA杂种中的RNA。它的主要功能是混合去除，特别是在R循环的情况下。RNA酶H2也存在于生活的所有分支中，可以起到类似的作用，但在切割嵌入DNA中的单个核糖核苷酸方面也具有特殊的功能。RNase H3存在于古细菌和细菌中，在序列和结构上与RNase H2密切相关，但具有类似RNase H1的生化特性。