The use of an RNA template to produce DNA, for integration into the host genome and exploitation of a host cell, is a strategy employed in the replication of retroid elements, such as the retroviruses and bacterial retrons. The enzyme catalysing polymerisation is an RNA-directed DNA-polymerase, or reverse trancriptase (RT) (<db_xref db="EC" dbkey="2.7.7.49"/>). Reverse transcriptase occurs in a variety of mobile elements, including retrotransposons, retroviruses, group II introns, bacterial msDNAs, hepadnaviruses, and caulimoviruses.<p>Retroviral reverse transcriptase is synthesised as part of the POL polyprotein that contains; an aspartyl protease, a reverse transcriptase, RNase H and integrase. POL polyprotein undergoes specific enzymatic cleavage to yield the mature proteins. The discovery of retroelements in the prokaryotes raises intriguing questions concerning their roles in bacteria and the origin and evolution of reverse transcriptases and whether the bacterial reverse transcriptases are older than eukaryotic reverse transcriptases [<cite idref="PUB00006358"/>].</p>