Abstract. Eukaryotic and prokaryotic genomes encode either Type I or Type II Ribonuclease H (RNH) which is important for processing RNA primers that prime DNA replication in almost all organisms. This review highlights the important role that Type I RNH plays in the life cycle of many retroelements, and its utility in tracing early events in retroelement evolution. Many retroelements utilize host genome-encoded RNH, but several lineages of retroelements, including some non-LTR retroposons and all LTR retrotransposons, encode their own RNH domains. Examination of these RNH domains suggests that all LTR retrotransposons acquired an enzymatically weak RNH domain that is missing an important catalytic residue found in all other RNH enzymes. We propose that this reduced activity is essential to ensure correct processing of the poly-Retrotransposable elements are so defined because they possess a reverse transcriptase (RT) enzyme, responsible for copying genetic information from RNA to DNA. It is widely accepted that such an enzymatic activity must have been involved in the early transition from the RNA world to one in which genetic information was primarily inherited via DNA. The homology of reverse transcriptases to extant viral RNAdependent RNA polymerases suggested their role as an evolutionary link between the RNA-dependent RNA polymerases and DNA-dependent DNA polymerases (Poch et al., 1989
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