Anthology of Human Repetitive DNA
279
protein, also called cORF, which is a func-
tional homolog of the HIV Rev protein.
K-Rev mediates nuclear export of the un-
spliced HERVK RNA by binding to both
the host Crm1 nuclear export factor and to
a cis-acting retroviral RNA target.
Mechanisms of retroviral evolution are
poorly understood since very few inter-
mediate forms of exogenous retroviruses
are available for analysis. Therefore, LTR
retroelements deposited in mammalian
genomes provide us with a unique op-
portunity
to
study
retrovirus-like
ele-
ments, which became extinct millions of
years ago.
3.3
DNA Transposons
All DNA transposons detected in the
human genome represent the so-called
cut-and-paste
category
since
they
use
excision from host DNA (cut) followed
by reinsertion (paste) as two basic steps
in their life cycle. Both steps are medi-
ated by the transposon-encoded enzyme
called
transposase
, which recognizes termi-
nal inverted repeats (TIRs; Fig. 11). The
transposon insertion is accompanied by
target site duplication.
The cut-and-paste process is thought to
be associated with the active replication
process. As shown in Fig. 12, the excision
is more likely in already replicated DNA
segments, whereas the insertion occurs
preferentially in nonreplicated segments.
On the basis of this model, the predicted
multiplication rate of cut-and-paste DNA
transposons is 1.5 per replication cycle; but
the real rate is expected to vary between 1
and 1.5.
To date, no active DNA transposons have
been found in the human genome. All
Transposase
Autonomous
Nonautonomous
TIR
TIR
TIR
[TSD]
[TSD]
[TSD]
[TSD]
TIR
DNA transposons
Fig. 11
Schematic structure of autonomous and nonautonomous DNA transposons.
Fig. 12
Relationship between
DNA transposition and host
replication.
Replication
Transposition
Replication is completed
(a)
(b)
(c)
(d)
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