274
Anthology of Human Repetitive DNA
LTR
ψ
gag
pro
pol
env
LTR
polyR
MA
RT
RH
IN
SU
TM
CA NC
(a)
SU
TM
NC
MA
CA
RNA
RT
IN
PRO
(b)
Fig. 8
The general structure of LTR retrotransposons. (a) Structural features of a provirus.
Black circles indicate target site duplications; LTR, long terminal repeat;
,pr
imerb
ind
ing
site;
9
, packaging signal;
gag
, capsid protein gene;
pro
, protease gene;
pol
, polymerase gene;
env
, envelope protein gene;
F
, deoxyuridine triphosphatase (dUTPase), which lies between
gag
and
pro
in some Class III retroviruses, and between
pol
and
env
in Class II retroviruses.
PPT stands for polypurine tract. The
pro
gene encodes the protease (PR), involved in the
processing of the
gag
-,
pol
-, and
env
-encoded proteins. The
pol
gene encodes the reverse
transcriptase (RT), ribonuclease H (RH), and integrase (IN). The
env
gene encodes the
surface (SU) and transmembrane (TM) proteins. Typically, the
gag
gene encodes the matrix
(MA), capsid (CA), and nuclear capsid (NC) structural proteins. (b) Schematic structure of a
retroviral particle. The viral envelope (green) is made of a host-cell-produced lipid bilayer. It
harbors multiple complexes formed by the
env
-encoded TM and SU components linked
together by disulFde bonds. The retroviral capsid formed by the CA surrounds two RNA
copies of the retrovirus, tRNAs, RT, IN, and PR. (See color plate p. xxiii).
Historically, LTR retrotransposons were
viewed as endogenous retrovirus-like el-
ements that did not encode the env
protein. Currently, they are considered
as nonautonomous retrovirus-derived el-
ements, and the originally introduced
distinction between retroviruses and LTR
retrotransposons is becoming obsolete.
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