Retroelements are mobile genetic entities that replicate via reverse transc
ription of a template RNA. A key component to the life cycle of these eleme
nts is the enzyme reverse transcriptase (RT), which copies the single-stran
ded genomic RNA of the element into a linear double-stranded DNA that is ul
timately integrated into the host genome by the element-encoded integrase.
RT is a multifunctional enzyme which possesses RNA-dependent and DNA-depend
ent DNA polymerase activities as well as RNase H activity that specifically
degrades the RNA strand of RNA-DNA duplexes. At some stages of the replica
tion a strand-displacement activity of RT is also necessary. All activities
are essential for the conversion of single-stranded genomic RNA into the d
ouble-stranded preintegrative DNA. This review focuses on the role of RT in
the different steps of the replication process of retroelements. The featu
res of retrotransposon replication which differ from the retroviral ones wi
ll be emphasized. In a second part of the review, the biochemical and enzym
atic properties of two newly characterized retrotransposon RTs will be desc
ribed. The role of the integrase domain in reverse transcriptase activity o
f some retroviral and retrotransposon RTs will be discussed.