tion in the germline [18]. Thus, mutations caused
by Tcl insertion into the unc-54 gene for the
heavy chain (HC) chain of myosin are frequently
found in Bergerac but never in Bristol [121].
Reversion of the mutated gene to the intact
unc-54+ is 1000 times more frequent in somatic
than in germline cells; the excised elements are
detected as extrachromosomal DNA [370]. In
addition to Tel, the related elements Tc2 to Tc5
are also found in C. elegans. Other elements
similar to Tel are TCbl (previously "Barney")
from Caenorhabditis briggsae, HBl from Drosophila melanogaster, and Uhu from Drosophila
heteroneura [47, 181].
The free-living nematode Panagrellus redivivus
shows high mutation rates in the DNA region that
is homologous to the unc-22 gene of C. elegans.
This is due to the insertion of a PAT-l element of
4.8 kb that appears in 10-50 copies at different
locations in various strains of P. redivivus. PAT-l
can probably be used as a vector for gene-transfer
experiments in C. elegans [266]. A retrovirus-like
element (Tas) , recently discovered in Ascaris
lumbricoides, consists of an internal sequence of
about 7 kb and two flanking LTRs of 256 bp. The
germline genome contains a total of 50 Tas elements at about 20 gene locations, with a 2: 1 distribution between the variants Tas-l and Tas-2.
Because only one-quarter of the Tas-l but all the
Tas-2 elements are eliminated during chromatin
reduction in somatic cell formation, Tas-2 elements are restricted to the germline [9].
2.3.5 The Retroposons of Vertebrates
Many sequences found in the mammalian
genome possess some of the characteristics of
retroviruses: long direct repeats (LTRs), internal
ORFs and short duplications of target sequences,
but only to a very limited extent do they represent
infectious viruses. The VL-30 and LTR-IS sequences of the house mouse and several other
Mus species [384] may be mentioned in this respect, together with the "mys" family of the North
American white-foot mouse Peromyscus leucopus;
this family is not found in Mus domesticus [475].
The human "THE1" repetitive sequences have
corresponding characteristics. These are viral
retroposons with a length of 2.3 kb and 350-bp
LTRs which duplicate a 5-bp sequence at the
insertion site. They code for a poly(A)-RNA of
about 2 kb that, however, shows no similarity to
known retrovirus products; thus, THEl is apparently transposed passively [104, 470]. The human
2.3.5 The Retroposons of Vertebrates
23
genome contains approximately 10 000 THE-l
sequences. The element is found in all primates,
although in only a few copies in the prosimians.
The THE-l of the pro simian galago is only in part
homologous to the human element; a new combination of existing genomic sequences apparently occurred in primate THE-l evolution, probably with the participation of a retrovirus [383].
The CRl element of the chicken, which is detectable in the neighbourhood of several genes, probably also belongs to the viral retroposons. This
consists of sequences of about 300 bp with short
terminal inverted repeats with opposite orientations, according to whether they are located 5' or
3' ofthe gene in question [424]. It is still not clear
whether sequences like CRl of the chicken or
OAX in the clawed frog Xenopus are really
capable of retroposition [470].
Two groups can be distinguished amongst the
non-viral retroposons of mammals: the short
SINEs with a length of 70-300 bp and usually
more than 10 5 copies; and the longer LINEs with
a length of more than 5 kb and never more than
10 4 copies. The SINEs consistently carry an internal Pol III promoter and mostly also a 3' poly(A)
tail. Only in the case of the C family of the ruminants can one find the more complicated sequence
(AGC)n instead of (A)n [470]. The best-known
SINE family is the Alu sequence of humans and
other primates; this owes its name to the restriction site for the endonuclease Alu I. In humans,
about 500000 Alu copies are present per genome
and make up 5-6 % of the DNA. Assuming a
regular distribution, an Alu sequence would be
found every 5-8 kb of human DNA; however,
they can occur in clusters. Thus, the 4826-bp
intron of the 5~ tubulin gene contains ten Alu,
and there are four in a 2.7-kb spacer between the
rRNA genes [158, 290]. The Alu elements are still
mobile. For example, in the ~-globin cluster of
the gorilla there exists an Alu element between bglobin and ~-g1obin, but this is not found in any
other primate; conversely, the intron 4 of the
human a-fetoprotein gene contains an Alu that is
not present in the gorilla [373, 440]. Alu-like
elements are widely distributed in mammals.
They show extensive sequence similarity to the
7SL RNA which is found in large amounts in the
cytoplasm and which is part of the signal recognition particle responsible for the membrane transport of secretory proteins; however, the central
part of the 7SL sequence is missing from the Alu
and Alu-like sequences (Fig. 2.5). The human
7SL RNA shows 87 % similarity to that of the
frog Xenopus laevis and 64 % to that of Droso-
by Tcl insertion into the unc-54 gene for the
heavy chain (HC) chain of myosin are frequently
found in Bergerac but never in Bristol [121].
Reversion of the mutated gene to the intact
unc-54+ is 1000 times more frequent in somatic
than in germline cells; the excised elements are
detected as extrachromosomal DNA [370]. In
addition to Tel, the related elements Tc2 to Tc5
are also found in C. elegans. Other elements
similar to Tel are TCbl (previously "Barney")
from Caenorhabditis briggsae, HBl from Drosophila melanogaster, and Uhu from Drosophila
heteroneura [47, 181].
The free-living nematode Panagrellus redivivus
shows high mutation rates in the DNA region that
is homologous to the unc-22 gene of C. elegans.
This is due to the insertion of a PAT-l element of
4.8 kb that appears in 10-50 copies at different
locations in various strains of P. redivivus. PAT-l
can probably be used as a vector for gene-transfer
experiments in C. elegans [266]. A retrovirus-like
element (Tas) , recently discovered in Ascaris
lumbricoides, consists of an internal sequence of
about 7 kb and two flanking LTRs of 256 bp. The
germline genome contains a total of 50 Tas elements at about 20 gene locations, with a 2: 1 distribution between the variants Tas-l and Tas-2.
Because only one-quarter of the Tas-l but all the
Tas-2 elements are eliminated during chromatin
reduction in somatic cell formation, Tas-2 elements are restricted to the germline [9].
2.3.5 The Retroposons of Vertebrates
Many sequences found in the mammalian
genome possess some of the characteristics of
retroviruses: long direct repeats (LTRs), internal
ORFs and short duplications of target sequences,
but only to a very limited extent do they represent
infectious viruses. The VL-30 and LTR-IS sequences of the house mouse and several other
Mus species [384] may be mentioned in this respect, together with the "mys" family of the North
American white-foot mouse Peromyscus leucopus;
this family is not found in Mus domesticus [475].
The human "THE1" repetitive sequences have
corresponding characteristics. These are viral
retroposons with a length of 2.3 kb and 350-bp
LTRs which duplicate a 5-bp sequence at the
insertion site. They code for a poly(A)-RNA of
about 2 kb that, however, shows no similarity to
known retrovirus products; thus, THEl is apparently transposed passively [104, 470]. The human
2.3.5 The Retroposons of Vertebrates
23
genome contains approximately 10 000 THE-l
sequences. The element is found in all primates,
although in only a few copies in the prosimians.
The THE-l of the pro simian galago is only in part
homologous to the human element; a new combination of existing genomic sequences apparently occurred in primate THE-l evolution, probably with the participation of a retrovirus [383].
The CRl element of the chicken, which is detectable in the neighbourhood of several genes, probably also belongs to the viral retroposons. This
consists of sequences of about 300 bp with short
terminal inverted repeats with opposite orientations, according to whether they are located 5' or
3' ofthe gene in question [424]. It is still not clear
whether sequences like CRl of the chicken or
OAX in the clawed frog Xenopus are really
capable of retroposition [470].
Two groups can be distinguished amongst the
non-viral retroposons of mammals: the short
SINEs with a length of 70-300 bp and usually
more than 10 5 copies; and the longer LINEs with
a length of more than 5 kb and never more than
10 4 copies. The SINEs consistently carry an internal Pol III promoter and mostly also a 3' poly(A)
tail. Only in the case of the C family of the ruminants can one find the more complicated sequence
(AGC)n instead of (A)n [470]. The best-known
SINE family is the Alu sequence of humans and
other primates; this owes its name to the restriction site for the endonuclease Alu I. In humans,
about 500000 Alu copies are present per genome
and make up 5-6 % of the DNA. Assuming a
regular distribution, an Alu sequence would be
found every 5-8 kb of human DNA; however,
they can occur in clusters. Thus, the 4826-bp
intron of the 5~ tubulin gene contains ten Alu,
and there are four in a 2.7-kb spacer between the
rRNA genes [158, 290]. The Alu elements are still
mobile. For example, in the ~-globin cluster of
the gorilla there exists an Alu element between bglobin and ~-g1obin, but this is not found in any
other primate; conversely, the intron 4 of the
human a-fetoprotein gene contains an Alu that is
not present in the gorilla [373, 440]. Alu-like
elements are widely distributed in mammals.
They show extensive sequence similarity to the
7SL RNA which is found in large amounts in the
cytoplasm and which is part of the signal recognition particle responsible for the membrane transport of secretory proteins; however, the central
part of the 7SL sequence is missing from the Alu
and Alu-like sequences (Fig. 2.5). The human
7SL RNA shows 87 % similarity to that of the
frog Xenopus laevis and 64 % to that of Droso-