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the signal recognition complex, the current dimeric Alu found in humans formed
from the merger of two monomeric units during early primate evolution. There
are approximately 1 million copies of Alu in the human genome (haploid),
constituting roughly 10% of the total genomic content (Lander
, 2001).
Alu elements do not encode the protein machinery required for their retro-
transposition. Instead, it has been experimentally demonstrated that they can
effectively commandeer L1 protein products to facilitate their proliferation
(Dewannieux
et al.
, 2003). While the ORF2 protein is sufficient for their retro-
transposition (Dewannieux
et al.
, 2003), the efficiency of Alu transposition can
be improved by the ORF1 expression (Wallace
et al.
et al.
, 2008a).
C. SVA
SVA elements arose more recently in the evolutionary timeline than the other
active elements, appearing later in primate evolution than Alu (Ostertag
et al.
,
2003; Wang
, 2005; classified as nonautonomous, they defy easy categoriza-
tion into existing schemas due to their intermediate size and chimeric nature).
These odd elements appear to be cobbled together from several distinct genomic
sources, including Alu-like sequence as well as sequence from viral envelope
genes and LTR regions (Ostertag
et al.
, 2003). Lacking protein-coding capacity,
they are nonautonomous elements that parasitize L1 to obtain the necessary
protein machinery for retrotransposition (Ostertag
et al.
, 2003). The presence of
several potential Pol-III termination sequences within their consensus sequences
argues for a Pol-II transcription and the involvement of the serendipitous
adjacent host promoters in driving SVA transcription has been reported
(Damert
et al.
et al.
, 2009).
D. Human endogenous retroviruses
HERVs are thought to be the ancestors of ancient retroviruses that infected the
germline and, subsequently, lost their ability to encode active envelope proteins
and move from host to host. Collectively, ERV copies make up
8% of the
human genome with human-specific ERVs (i.e., HERVs) constituting a smaller
fraction (Lander
, 2001). Due to their ability to be brought in through the
infection of the germline by exogenous retroviruses, HERVs have been intro-
duced and reintroduced a multitude of times during human evolution. While
their rate of proliferation in modern human germlines appears to be limited, with
only a modest number of polymorphic locations being identified (Turner
et al.
,
2001), the activity of their remnant promoters has significant biological con-
sequences, having been often adopted in several instances for host genome
et al.
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