Biology Reference
In-Depth Information
The initiation of retrotransposition in the nucleus involves recognition
of an EN target site within the host genomic DNA by the L1 ORF2 followed by
the first-strand DNA cleavage by the L1 ORF2 EN that produces a DNA nick at
the EN site (Cost and Boeke, 1998; Feng
, 1996; Jurka, 1997). Even though
the consensus L1 EN site is deduced to be 5
0
-TTTTAA-3
0
, some of the sites with
a single nucleotide substitution within this sequence are known to be efficiently
used by the L1 EN (Jurka, 1997; Szak
et al.
, 2002). The cleavage of genomic
DNA at the T-A junction is proposed to generate a single strand (ss), 5
0
-TTTT-
3
0
, DNA available for a noncovalent interaction with the polyA tail at the 3
0
end
of the L1 mRNA (or SINE and SVA RNAs; Feng
et al.
,
2002). Free 3
0
end of the ss genomic DNA serves as a primer for the L1 RT to
synthesize first-strand L1 cDNA (Martin
et al.
, 1996; Symer
et al.
, 1998; Piskareva and
Schmatchenko, 2006). This process is known as TPRT and is common among
retroelements from species as diverse as the silk worm,
et al.
Bombyx mori
, and human,
Homo sapiens
, 2002). The remain-
ing steps of the retrotransposition process that include second strand L1 DNA
synthesis (Piskareva and Schmatchenko, 2006), removal of the RNA template,
and covalent connection of the
(Christensen and Eickbush, 2005; Cost
et al.
L1 (or Alu and SVA) insert with
genomic DNA at the site of integration continue to be very poorly characterized.
It is speculated that cellular DNA repair proteins or other cellular factors are
likely to assist in the completion of some or all of these steps. This assumption is
based on the apparent lack of certain enzymatic functions within L1 proteins,
such as ligase activity, that would be required to carry out these steps. It is further
based on the recent discoveries of the opposing effects of cellular proteins from
various DNA repair pathways on the efficiency of L1 retrotransposition (Gasior
et al.
de novo
, 2009). The final product of retrotransposition
contains distinct structural characteristics such as L1, SINE, SVA, or cellular
transcript sequences flanked by the EN recognition site known as the target site
duplication (TSD). These signature features unequivocally identify L1 machinery
involvement in the generation of integration events.
Even though L1, SINEs, and SVA elements rely completely on the L1
retrotransposition apparatus for their mobilization, different elements exhibit dis-
tinct differences between the retrotransposition requirements. One of these pre-
requisites is the dependence or the lack of it on the L1 ORF1 protein (Dewannieux
et al.
, 2007, 2008; Suzuki
et al.
,1996). As mentioned above, in contrast to L1, which
absolutely requires participation of the functional ORF1 protein in addition to the
ORF2 protein for successful retrotransposition, Alu elements can be efficiently
mobilized by the L1 ORF2 alone (Dewannieux
,2003;Moran
et al.
,1996).
While the presence of the L1 ORF1 protein slightly stimulates Alu retrotransposi-
tion in tissue culture (Wallace
et al.
, 2003; Moran
et al.
, 2008a), this modest effect supports the
observation that Alu retrotransposition is largely independent of the L1 ORF1.
The proposed explanations for this discrepancy include the difference in the origin
et al.
