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Functional characterization of RNase H1 proteins in Arabidopsis thaliana

RNase H1 is an endonuclease specific towards RNA:DNA hybrids. Members of this protein family are present in most living organisms and are essential for removing RNA that base pairs with DNA. It prevents detrimental effects of RNA:DNA hybrids and is involved in several biological processes. We show that Arabidopsis thaliana contains four RNase H1-like proteins originating from two gene duplication events and alternative splicing. These proteins have the canonical RNase H1 activity, which requires at least four ribonucleotides for activity. Two of those proteins are nuclear, one is localized to mitochondria and one to plastids. While the nuclear RNases H1 are dispensable, the presence of at least one organellar RNase H1 is required for embryonic development. The plastid protein RNH1C affects plastid DNA copy number and sensitivity to hydroxyurea. This indicates that three genomes present in each plant cell are served by at least one specialized RNase H1 protein.