22
2 Nucleic Acids and Nuclear Proteins
ses a nucleic acid-binding protein, a protease, an
integrase and a reverse transcriptase. In cultured
Drosophila cells, one finds retrovirus-like particles, with copia RNA and transcriptase activity
[490].
Foldback (FB) elements in the genus Drosophila are found only in the species groups of D.
melanogaster, D. montium and D. takahashi, but
otherwise are widely distributed in the eukaryotes. They carry at both ends a long inverted
repeat that results in the folding of the denatured
element. In contrast to the complex terminal
repeats, there is a tandem sequence of 31-bp
repeats towards the middle [397]. Two flanking
FB elements can mobilize large DNA sections
and transport them to other genome positions;
this has been shown for the loci "white" (w),
"roughest" (rst) and "no-ocelli" (noc) in Drosophila [48, 71]. The element "mariner", 1.3 kb
long and with terminal inverted repeats, is also
confined to particular species of the Drosophila
genus, where it produces, for example, eyecolour mutations. It is found in 20-30 copies in
the genome of Drosophila mauritiana, in lower
numbers in, e.g., D. yakuba and D. simulans,
and not at all in D. melanogaster and D. erecta
[53].
P elements have spread throughout the genus
Drosophila by some as yet unknown mechanism.
The first P elements were observed in the 1950s in
North American populations of D. melanogaster.
In the following two decades, these sequences
spread into Europe, Asia and Australia, so that
today P-free (M) strains are to be found in only a
few places. Contrary to earlier assumptions, P
elements are also present in other species of Drosophila, not as it happens in those most closely
related to D. melanogaster but in the distantly
related species group based on D. willistoni. This
has led to the idea that P elements were transferred from a species of the D. willistoni group to D.
melanogaster through some sort of infection process [103, 172]. P elements can be used as vectors
to transfer foreign sequences into the genome of
D. melanogaster [95]. Complete P elements,
which are only transposed in cells of the germline,
are 2.9 kb long, carry terminal repeats of 31 bp
but no LTRs, and contain four open reading frames (ORFO to ORF3) that together code for the
87-kDa enzyme (transposase) necessary for the
transposition in germline cells. In somatic cells,
splicing of ORFO, ORFl and ORF2 results in a
66-kDa protein that acts as a transposition repressor [95, 361]. Crossing of a P male with an M
female drastically increases the P-element transposition rate; this causes multiple mutations and
chromosomal rearrangements and leads to over
90 % sterility. This effect is known as "hybrid dysgenesis". Reciprocal crosses between M males
and P females, or intercrossing of P males and
females, does not increase the P-element transposition rate; this stems most probably from the
large amount of the 66-kDa protein that is found
in the eggs of P females [95].
Hybrid dysgenesis can also be induced in D.
melanogaster by the inducer (I) factors; these are
5.4 kb long, without terminal repeats but have a
3' poly(A) sequence. They contain two open
reading frames: ORF2, which encodes a reverse
transcriptase, and ORF1, which encodes a
cysteine-rich protein similar to the retroviral gag
protein. During the transposition of I factors, an
RNA is produced through polymerase II activity
and forms a ribonucleoprotein complex with the
products of ORFl and ORF2. The genomes of
flies from inductive I strains contain incomplete
and about 15 complete I factors; only defective I
factors are found in the responsive R strains.
These factors are widely spread in the Drosophila
genus, but have only relatively recently been
transferred to D. melanogaster. The I factors of
the closely related species D. simulans are more
similar to those from D. melanogaster than those
from D. teissieri [1, 454]. I factors closely
resemble the LINEs of mammals, and this is true
for a series of further insect retroposons such as
F-, G-, and doc elements and jockey in Drosophila, or TIAg in Anopheles gambiae [33, 301,
325].
In comparison with those of Drosophila, the
mobile elements of other invertebrates have been
investigated to only a limited extent. The single
exception is the Tel element of Caenorhabditis
elegans; this small nematode can be easily cultured and has been extensively investigated at the
molecular biology level. The Tcl element has only
1610 bp with terminal inverted repeats of 54 bp,
and produces a 2-bp duplication of the target
sequence. Tcl can be transposed directly; the
ORF of 273 triplets codes for a transposase carrying an N-terminal DNA-binding sequence [388].
Only 25 copies of the element are present in the
Bristol strain of C. elegans, compared with the
250 copies in the strains Bergerac and DH424.
Contrary to previous opinion, the elements are
not completely identical; 20 of the 250 elements
in Bergerac and one in Bristol have a cutting site
for the HindIII restriction enzyme; this is missing
from all the others [369]. In most strains with few
Tel copies, there is no or negligible transposi-
2 Nucleic Acids and Nuclear Proteins
ses a nucleic acid-binding protein, a protease, an
integrase and a reverse transcriptase. In cultured
Drosophila cells, one finds retrovirus-like particles, with copia RNA and transcriptase activity
[490].
Foldback (FB) elements in the genus Drosophila are found only in the species groups of D.
melanogaster, D. montium and D. takahashi, but
otherwise are widely distributed in the eukaryotes. They carry at both ends a long inverted
repeat that results in the folding of the denatured
element. In contrast to the complex terminal
repeats, there is a tandem sequence of 31-bp
repeats towards the middle [397]. Two flanking
FB elements can mobilize large DNA sections
and transport them to other genome positions;
this has been shown for the loci "white" (w),
"roughest" (rst) and "no-ocelli" (noc) in Drosophila [48, 71]. The element "mariner", 1.3 kb
long and with terminal inverted repeats, is also
confined to particular species of the Drosophila
genus, where it produces, for example, eyecolour mutations. It is found in 20-30 copies in
the genome of Drosophila mauritiana, in lower
numbers in, e.g., D. yakuba and D. simulans,
and not at all in D. melanogaster and D. erecta
[53].
P elements have spread throughout the genus
Drosophila by some as yet unknown mechanism.
The first P elements were observed in the 1950s in
North American populations of D. melanogaster.
In the following two decades, these sequences
spread into Europe, Asia and Australia, so that
today P-free (M) strains are to be found in only a
few places. Contrary to earlier assumptions, P
elements are also present in other species of Drosophila, not as it happens in those most closely
related to D. melanogaster but in the distantly
related species group based on D. willistoni. This
has led to the idea that P elements were transferred from a species of the D. willistoni group to D.
melanogaster through some sort of infection process [103, 172]. P elements can be used as vectors
to transfer foreign sequences into the genome of
D. melanogaster [95]. Complete P elements,
which are only transposed in cells of the germline,
are 2.9 kb long, carry terminal repeats of 31 bp
but no LTRs, and contain four open reading frames (ORFO to ORF3) that together code for the
87-kDa enzyme (transposase) necessary for the
transposition in germline cells. In somatic cells,
splicing of ORFO, ORFl and ORF2 results in a
66-kDa protein that acts as a transposition repressor [95, 361]. Crossing of a P male with an M
female drastically increases the P-element transposition rate; this causes multiple mutations and
chromosomal rearrangements and leads to over
90 % sterility. This effect is known as "hybrid dysgenesis". Reciprocal crosses between M males
and P females, or intercrossing of P males and
females, does not increase the P-element transposition rate; this stems most probably from the
large amount of the 66-kDa protein that is found
in the eggs of P females [95].
Hybrid dysgenesis can also be induced in D.
melanogaster by the inducer (I) factors; these are
5.4 kb long, without terminal repeats but have a
3' poly(A) sequence. They contain two open
reading frames: ORF2, which encodes a reverse
transcriptase, and ORF1, which encodes a
cysteine-rich protein similar to the retroviral gag
protein. During the transposition of I factors, an
RNA is produced through polymerase II activity
and forms a ribonucleoprotein complex with the
products of ORFl and ORF2. The genomes of
flies from inductive I strains contain incomplete
and about 15 complete I factors; only defective I
factors are found in the responsive R strains.
These factors are widely spread in the Drosophila
genus, but have only relatively recently been
transferred to D. melanogaster. The I factors of
the closely related species D. simulans are more
similar to those from D. melanogaster than those
from D. teissieri [1, 454]. I factors closely
resemble the LINEs of mammals, and this is true
for a series of further insect retroposons such as
F-, G-, and doc elements and jockey in Drosophila, or TIAg in Anopheles gambiae [33, 301,
325].
In comparison with those of Drosophila, the
mobile elements of other invertebrates have been
investigated to only a limited extent. The single
exception is the Tel element of Caenorhabditis
elegans; this small nematode can be easily cultured and has been extensively investigated at the
molecular biology level. The Tcl element has only
1610 bp with terminal inverted repeats of 54 bp,
and produces a 2-bp duplication of the target
sequence. Tcl can be transposed directly; the
ORF of 273 triplets codes for a transposase carrying an N-terminal DNA-binding sequence [388].
Only 25 copies of the element are present in the
Bristol strain of C. elegans, compared with the
250 copies in the strains Bergerac and DH424.
Contrary to previous opinion, the elements are
not completely identical; 20 of the 250 elements
in Bergerac and one in Bristol have a cutting site
for the HindIII restriction enzyme; this is missing
from all the others [369]. In most strains with few
Tel copies, there is no or negligible transposi-