enzyme present in the virion of all retro-viruses and in cells during replication of hepadnaviruses. It copies single-stranded RNA into DNA, and has ribonuclease H activity which digests RNA present in RNA-DNA hybrid molecules. Like DNA polymerase, the enzyme requires a primer for DNA synthesis. The natural primer in retroviruses is a species of tRNA bound some 100 or so nucleotides from the 5' end of the genome RNA; the primer varies according to the virus species. It is tRNATrp for avian sarcoma virus and probably for all avian leukosis viruses; tRNAPro for Moloney leukemia virus, all other murine leukemia viruses which have been examined, simian sarcoma virus and avian reticuloendotheliosis virus; tRNALys3 for Mouse mammary tumor virus; and tRNALys1,2 for HIV. Hepadnavirus replication involves a reverse transcription step primed by a polypeptide. During establishment of retrovirus infection, the end-product of RNA-dependent DNA polymerase activity is a linear double-stranded DNA molecule containing terminal repeats which subsequently becomes integrated into the cell genome. The enzyme has been purified free of the natural template and in this form is widely used for genetic manipulation and nucleic acid sequencing. The crystal structure shows that the enzyme is a het-erodimer of full-length reverse transcriptase (p66) and a cleavage product (p51) that lacks the C-terminal ribonuclease H domain. Synonym: reverse transcriptase.
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