Natural Transposon: Dmel\mdg1
Natural Transposon: Dmel\mdg1
25 (Ilyin et al.)
25 in euchromatin of Release 3 genome annotation, of which 13 are full length.
TE copies retrieved from release 5.1 of the D. melanogaster genome.:42
Expression is enriched in embryonic gonads.
Transposable elements can be used to reveal cross-over events.
No transposition was detected in progeny after heat shock of parents.
A lower proportion of copia, mdg1 and 412 element insertion sites on the X chromosome, from various populations of D.melanogaster and D.simulans, in comparison with autosomes suggests that selection against the detrimental effects of TE insertions in the major force containing TE copies in populations.
The behaviour of the retrotransposons copia, Dsim\copia and mdg1 has been analysed in hybrids between D.melanogaster and D.simulans. No somatic transposition events were detected in hybrid larvae.
Study of TE distribution (P-element, hobo, I-element, copia, mdg1, mdg3, 412, 297 and roo) along chromosome arms shows no global tendency for the TE site occupancy frequency to negatively follow the recombination rate, except for the 3L arm. The tendency for TE insertion number to increase from base to tip of some chromosome arms is simply explicable by a positive relationship with DNA content along the chromosomes. So for all TEs, except hobo, there is no relationship between distribution of TE insertion numbers weighted by DNA content and recombination rate. hobo insertion site number is positively correlated with recombination rate.
Endogenous transposable elements show more instability in sublines injected with exogenous viral particles than in transgenic sublines containing a foreign viral insert, all transposable elements are not equally sensitive to such genomic stress.
Transcriptional analysis of mdg1-Ecol\CAT fusion constructs indicates that mdg1 can be transcribed by both RNA polymerases II and III.
The distribution of mdg1 elements in heterochromatin has been studied by in situ hybridisation to mitotic chromosomes.
Experiments designed to compare the insertion patterns of copia and mdg1 revealed that crossing to marker strains led to heterogeneity in insertion patterns of the copia elements, with no significant polymorphism of the mdg1 insertions.
Spontaneous insertions and excisions of mdg1, copia, 412 and roo (excisions are outnumbered by insertions) occur during 65 generations of mass mating under laboratory conditions. Their contribution into variation for transposable element location does not seem great.
The distribution of a number of transposable elements has been studied in 10 Harwich mutation accumulation lines.
The distribution of transposable elements within heterochromatin indicates that they are major structural components of the heterochromatin.
The distribution of mdg1 elements across the chromosomes has been analysed in individuals from a natural population of D.melanogaster.
Estimating the genomic numbers of transposable elements demonstrates many families of element are over-represented in heterochromatin.
The spatial and temporal expression patterns of fifteen families of retrotransposons are analysed during embryogenesis and are found to be conserved. Results suggest that all families carry cis-acting elements that control their spatial and temporal expression patterns.
The insertion patterns of mdg1 and copia are sufficiently modified to allow the unambiguous detection of an alien genome income.
60kb repeats located in the distal heterochromatin of the X chromosome have been cloned. These regions, designated as SCLRs, are comprised of the following types of repeated elements: SteXh, copia-like elements (mdg1 elements, aurora-elements and GATE elements), LINE-elements (G-elements and R1-elements), and bb fragments. There are approximately 9 SCLR copies per haploid genome, with a twofold variation in copy number between different fly stocks.
Element copy numbers on inversion and standard chromosomes has been determined. The copy number is significantly higher within low frequency inversions than within the corresponding standard chromosome regions.
Evolutionary history of mobile and nonmobile mdg1 elements in the genome is determined.
Polymorphism of transposable elements in inbred lines has been examined: P-element, gypsy, jockey, I-element, mdg1, 412, mdg3 and 297 sites are largely stable, whereas roo and copia sites are polymorphic.
During the course of experiments with genetically unstable MS strains gypsy elements were observed to transpose whereas mdg1 and 412 sites in the X chromosome were unchanged.
Numbers of mdg1, mdg3, gypsy and copia have been studied in several strains of D.melanogaster and D.simulans. Mean number of mdg1 and copia sites are drastically reduced in D.simulans. Majority of mdg1 and copia sites, and one third of mdg3 sites, are in hot spots for insertion, particularly in D.simulans. Southern blot analysis indicates that the majority of mdg1 and copia are in the euchromatin of D.melanogaster but the heterochromatin of D.simulans.
Non mobile mdg1 located in D.melanogaster heterochromatin was sequenced and compared with the transposable version of mdg1. Results suggested that the evolution of mdg1 subfamilies occurred under selective pressure on the ability to transpose. The divergence of the left and right LTRs of heterochromatic aurora-element and mdg1 elements indicates that aurora-element has been at its heterochromatic location for 0-0.15Myr and mdg1 for 0-0.7Myr.
One substock of inbred lines shows considerable heterogeneity of insertion sites of copia (frequency of insertions is 12% per haploid genome per generation) whereas mdg1, 412, mdg3, gypsy, 297 and HMS-Beagle were stable in all stocks examined.
Sequences required for correct and precise initiation of mdg1 RNA synthesis have been determined using mdg1-Ecol\CAT regulatory fusion constructs.
Transient expression of mdg1 deletion constructs identifies sites of 3'-end processing in the leader region of the transcribed RNA.
The mdg1 element has been cloned and sequenced. The element contains two long partially overlapping reading frames (ORFs), ORF1 and ORF2, which encode the proteins required for reverse transcription. The mdg1 element contains unusually long leader and terminal regions. The leader region contains two short open reading frames which are separated from each other and the long ORFs by long oligo(dA) sequences. One of these short ORFs encodes a protein with a zinc-binding region. The mdg1 element shows considerable homology with the 412 element.
The mdg1 element contains transcription termination sites containing long blocks of oligo(dA) in the leader region of the element, 1kb downstream of the transcription start site. Transient expression of deletion mutants shows that a small open reading frame (ORF) in the leader region can be translated, and suggests transcription reinitiation may occur during the process of reading the main ORF of the mdg1 element.
The distribution of a number of transposable elements, including mdg1 elements, in a D.melanogaster laboratory strain with a high frequency of spontaneous mutations and its derivatives, has been studied.
Two regions in the mdg1 element can specifically bind nuclear proteins of D.melanogaster. The first region is 1kb downstream of the transcription start site, and the second region is localised near the 3' LTR. The two regions are recognised by different proteins and may be involved in the regulation of mdg1 transcription. Binding of proteins to the first region can be suppressed by adding 412 element DNA.
A considerable proportion of mdg1 elements are located in heterochromatic chromosome regions. Many of these heterochromatic mdg1 elements are inserted into a non-mobile heterochromatic moderate repeat, named the HMR-element. HMR-elements along with the mdg1 copies inserted in them are under-replicated in polytene chromosomes.
The genomic distribution of transposable elements in somatic tissues and during development is homogeneous.
First described by Ilyin et al. (1978) and Georgiev et al. (1978) as being complementary to abundant poly(A)+ RNA. The sequence of the LTR shown here is the reverse complement of that published by Kulguskin et al. (1981) and the map in Lindsley, Zimm, 1992: 1103 is the reverse of that published by Ilyin et al. (1978). The direction of major transcription is left to right (Ilyin et al.). Fourteen of the eighteen bases of the putative primer binding sites of mdg1 and 412 elements are identical, as are the 27 bases adjacent to their left-hand LTRs (Will et al., 1981). Yuki et al. (1986) have identified an arginine tRNA as being the probable primer for reverse transcription of both mdg1 and 412 RNAs.