264
Anthology of Human Repetitive DNA
RNA from the nucleus to the cytoplasm,
translation and formation of ribonucleo-
protein particles, which are transported
back into the nucleus, reverse transcrip-
tion, and integration into the genome. L1
elements lack splice signals and there-
fore, L1 mRNA is bi-cistronic with two
open reading frames separated by a short
noncoding spacer. Translation of ORF1 is
likely to be initiated at the 5
0
UTR by ri-
bosomal scanning. ORF2 translation may
be initiated by an internal ribosomal en-
try site (IRES) or by the so-called
ribosomal
shunting
or
jumping
. Also, it is unclear how
the L1-encoded proteins enter the nucleus.
Usually, nuclear export of proteins as large
as those encoded by ORF1 and ORF2 is
driven by speci±c amino acid sequences
called
nuclear localization signals
.Asthere
are no known nuclear localization signals
encoded by either ORF1 or ORF2 proteins,
the mechanism involved in their entry into
the nucleus remains a mystery. It is even
unclear if the ORF1 translation products
ever enter the nuclear compartment.
The model of reverse transcription and
integration of L1 into the genome invokes
a coupled process known as
target-primed
reverse transcription
(TPRT). According to
this model, the APE domain of the
ORF2 protein recognizes a 6-bp target
site described by the TTAAAA consensus
sequence, and nicks the second DNA
strand between the bases complementary
to
TT
and
AAAA
(Fig. 4).
The
free
3
0
-OH end of the nick is then used
for priming reverse transcription of L1
RNA by the ORF2 reverse transcriptase,
starting from the 3
0
polyA tail. Eventually,
a second nick is created on the ±rst
strand, preferably
15 bp downstream
of the ±rst nicking site, leading to a
staggered break and, after integration, to
the formation of target site duplications
(TSDs) flanking the integrated sequence.
The typical size of TSDs generated by
L1 retrotransposition is
15 bp, but the
size may vary and sometimes even short
deletions of target sequences are observed.
The ±nal integration steps, including
5
3
3
5
5
3
5
3
3
5
5
3
3
5
3
5
ACT
TTAAAA
CGTATGT
TYTN
AAA
TGA
AATTTT
GCATACA
ARAN
TTT
ACT
TTAAAA
CGTATGT
TYTN
AAA
TGA
AA
TTT
TTTT
GCATACA
ARAN
AAAA
L1 RNA
ACT
TTAAAA
CGTATGT
TYTN
TGA
AA
L1
cDNA
ACT
TTAAAA
CGTATGT
TYTN
TGA
AATTTT
GCATACA
ARAN
TTTT
GCATACA
ARAN
TTT
AAAA
CGTATGT
TYTN
AAA
(a)
(c)
(d)
(b)
L1
TSD
TSD
TTTT
GCATACA
ARAN
TTT
AAA
Fig. 4
The target-primed reverse transcription (TPRT) mechanism. (a) The
L1 endonuclease cleaves the antiparallel DNA strand between 3
0
AA and 3
0
TTTT, and exposes a free 3
0
hydroxyl group at the nick, serving as a primer for
reverse transcription of L1 mRNA. (b) The second nicking occurs
15
nucleotides from the Frst nicking site, preceded by a TYTN consensus
sequence. (c) The remaining steps include elimination of L1 RNA, DNA
synthesis, and repair, probably mediated by the host enzymes. (d) Integrated
L1 retroelement and its flanking sites.
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