Biology Reference
In-Depth Information
ERV-FTD
HERV-1
ERV-9
ERV-FRD
BaEV
GaLV
AKV
MoMuLV
S71-TK6
HERV-E
HERV-K (HML-1)
ERV-MLN (HML-4)
MMTV
IAP-M
C-type
B-type
Figure 8.4.
Phylogenetic analysis of mammalian endogenous retroviral
pol
sequences
(after Leib-Mösch and Seiforth, 1996). HERV: Human endogenous retroviral elements
ERV: Endogenous retrovirus BaEV: Baboon endogenous virus GaLV: Gibbon ape
leukemia virus AKV: Endogenous murine leukemia virus MoMuLV: Moloney murine
leukemia virus MMTV: Mouse mammary tumor virus IAP-M: Murine intracisternal A-
type particles.
Transposons.
Transposon-like THE-1 repeats, which lack any obvious homology
to retroviral sequences, have been found in the deletion-prone intron 43 of the
dystrophin (
DMD
; Xp21.2-p21.3) gene (Pizzuti
et al.
, 1992), the human blood
group
GC
(4q12) gene (Witke
et al.
, 1993) and the 3
untranslated region of the
human calmodulin-related protein (
CALML1
; 7p13-pter) gene (Deka
et al.
,
1988b). A cluster of three THE-1 repeats located in a 26 kb region of intron 7 in
the human
DMD
gene has arisen by three independent insertion events
(McNaughton
et al.
, 1993, 1995). There is some evidence to support the hypothe-
sis that the insertion of these elements has occurred at preferred target sites (Deka
et al.
, 1988a).
A pseudoautosomal gene sequence (
Tramp
) has recently been isolated which
encodes within its single exon a protein with homology to transposases (enzymes
that mediate transposition) of the Ac family (Esposito
et al
., 1999). It is as yet
unclear whether the Tramp protein has been involved in the transposition of
other transposable elements or if it has instead become specialized for a novel cel-
lular function. The centromeric protein CENP-B (
CENPB
; 20p13) may also rep-
resent an example of a transposase-encoded protein which has acquired a cellular
function. This protein binds to the CENP-B box (TTCGNNNNANNCGGG)
sequence in the alpha satellite DNA of human centromeres and has sequence sim-
ilarity to the
pogo
family of transposases which includes the
Tigger
elements
(Kipling and Warburton, 1997). Since CENP-B has nicking activity, it may pro-
mote homologous recombination and could have contributed to the species-spe-
cific patterns of evolution of satellite repeats.
