Mobile dispersed genetic element MDG1 of Drosophila melanogaster: structural organization

Unequal crossing-over associated with asymmetrical synapsis between nomadic elements in the Drosophila melanogaster genome

The molecular structure of reciprocal duplications and deficiencies produced by unequal crossing-over at the white (w) locus of Drosophila melanogaster females heterozygous for the alleles w(a) and w(a4) has been examined. A transposable, copia-like element is found at the rearrangement breakpoints. Further characterization indicates that asymmetrical pairing between two copies of this element, which are at least 60 kilobases apart in the parental chromosomes, followed by a crossover within the paired elements, is responsible for the duplication and deficiencies observed. The frequency of these events is high compared with normal homologous exchange, implying that synaptic pairing during meiosis must be sufficiently flexible as to allow efficient recognition of sequences located in nonidentical positions on homologous chromosomes. These results suggest a possible mechanism for the generation of tandem duplications in eukaryotic organisms.

Spatially regulated expression of retrovirus-like transposons during Drosophila melanogaster embryogenesis

Over twenty distinct families of long terminal direct repeat (LTR)-containing retrotransposons have been identified in Drosophila melanogaster. While there have been extensive analyses of retrotransposon transcription in cultured cells, there have been few studies of the spatial expression of retrotransposons during normal development. Here we report a detailed analysis of the spatial expression patterns of fifteen families of retrotransposons during Drosophila melanogaster embryogenesis (17.6, 297, 412, 1731, 3S18, blood, copia, gypsy, HMS Beagle, Kermit/flea, mdg1, mdg3, opus, roo/B104 and springer). In each case, analyses were carried out in from two to four wild-type strains. Since the chromosomal insertion sites of any particular family of retrotransposons vary widely among wild-type strains, a spatial expression pattern that is conserved among strains is likely to have been generated through interaction of host transcription factors with cis-regulatory elements resident in the retrotransposons themselves. All fifteen families of retrotransposons showed conserved patterns of spatially and temporally regulated expression during embryogenesis. These results suggest that all families of retrotransposons carry cis-acting elements that control their spatial and temporal expression patterns. Thus, transposition of a retrotransposon into or near a particular host gene-possibly followed by an excision event leaving behind the retrotransposon's cis-regulatory sequences-might impose novel developmental control on such a host gene. Such a mechanism would serve to confer evolutionarily significant alterations in the spatio-temporal control of gene expression.

Transposable elements in commercially useful insects: I. Southern hybridization study of silkworms and honeybees using Drosophila probes

As a first step in surveying transposable elements in silkworms and honeybees, hybridization analyses were carried out using 16 known families of Drosophila transposable elements as probes. jockey and G were the only transposable elements that hybridized with genomic DNA of either honeybees or silkworms under the conditions of this study. jockey hybridized with genomic DNA of both European honeybees (Apis mellifera) and silkworms (Bombyx mori and Antheraea yamamai) and showed significant bands in Southern blots. Banding patterns were highly polymorphic. jockey did not, however, hybridize with any strains of the Asian honeybee (A. cerana). G elements showed a faint signal with the Asian honeybee, but not with any other insects tested. The results suggest that, even though it has some limitations, this approach can be used in practice as a first preliminary step in surveys for the presence of transposable elements in organisms which do not have good genetic information.

Distribution of Drosophila melanogaster transposable element sequences in species of the obscura group

Fifteen species belonging to the obscura group of the genus Drosophila were screened for sequences homologous to Drosophila melanogaster transposable elements (TEs) as an initial step in the examination of the possible occurrence of TEs at chromosomal inversion breakpoints. Blots of genomic DNAs from species of the obscura group were hybridized at three different stringencies with 14 probes representing the major families of TEs described in D. melanogaster. The probe DNAs included copia, gypsy, 412, 297, mdg1, mdg3, 3S18, F, G, I, jockey, P, hobo, and FB3. D. melanogaster TEs were not well represented in the species of the obscura group analyzed. The TEs that were observed generally exhibited heterogeneous distributions, with the exception of F, gypsy and 412 which were ubiquitous, and 297, G, Sancho 2, hobo and FB which were not detected.

Leader region of mdg1 Drosophila retrotransposon RNA contains 3'-end processing sites

Transient expression of the mdg1 deletion mutants revealed sites of 3'-end processing in the leader region of the transcribed RNA. The efficiency of the processing is regulated in different types of cells. The sequences within the mdg1 body and the 3'-LTR are involved in its regulation. We have also shown, that one of the small open reading frames in the mdg1 leader region in principle might be translated.

Genomic distribution of copia-like transposable elements in somatic tissues and during development of Drosophila melanogaster

The genomic distribution of elements of the copia, 412, B 104, mdg 1, mdg 4 and 1731 transposon families was compared by the Southern technique in DNA preparations extracted from brains, salivary glands and adult flies of two related Drosophila lines. The copia, 412 and mdg 1 sequences were also probed in DNA from sperm, embryos, and 1st and 2nd instar larvae. The homogeneity of the patterns observed shows that somatic transposition is unlikely to occur frequently. A correlation between mobility and the euchromatic or heterochromatic location of transposable elements is discussed. In addition, an explanation of the variable band intensities of transposable elements in Southern autoradiographs is proposed.

On the transposition of copia-like nomadic elements in cultured Drosophila cells

We studied the stability of the genomic distribution of six retrotransposon families in long-term and short-term cultures of Drosophila cells. In a subclone derived from Kc cells, no significant rearrangements were detected over an 8 year period. On the contrary, extensive reshuffling and amplification of transposon families were observed in recently established cell lines. These results show that in cultured Drosophila cells transposition appears to be restricted to the transition from the embryo to continuous cell lines.

The steps of reverse transcription of Drosophila mobile dispersed genetic elements and U3-R-U5 structure of their LTRs

Reverse transcription intermediate forms (minus and plus strong-stop DNA) are detected in Drosophila melanogaster cultured cells for mobile dispersed genetic elements mdg1, mdg3, and mdg4 (gypsy). The mdg elements studied possess a common mechanism of reverse transcription, despite their structural differences, and the comparative analysis of intermediate forms proves that mdg elements pass the same stages of reverse transcription as retroviruses. The length of minus strong-stop DNA that locates the RNA start site coincides with the data obtained from S1 nuclease analysis of transcription initiation. S1 analysis has also revealed that mdg LTRs have a U3-R-U5 structure analogous to that of retroviral LTRs. Transcription of mdg1, mdg3, and mdg4 is initiated within or immediately after the same sequence TCAGTPy. Neither the TATA box nor the CAAT box can be found at their characteristic positions upstream of the 5' ends of mRNAs.

The unusual structure of heat shock locus 2-48B in Drosophila hydei

We have previously isolated a 500 bp-long cDNA clone, NO9-15, which is derived from a nuclear transcript originating from the heat shock locus 2-48B of Drosophila hydei (Peters et al. 1982). Sequence analysis shows that this clone carries 4 complete copies and 1 partial copy of a 115 bp repeat unit. The repeats are closely homologous with a maximal sequence divergence of about 10%. The sequence does not contain an open reading frame. The genomic organization of heat shock locus 2-48B, as probed with the cloned cDNA sequence NO9-15, is highly polymorphic. Four different allelic arrangements have been found in different inbred strains. A number of genomic clones isolated from region 2-48B, both in phage lambda and in cosmid vectors, all differ in length, mainly due to varying numbers of the NO9-15 repeat unit. These differences are found primarily in the proximal region of the locus. The transcribed region of these clones includes the distal sequence flanking the NO9-15 repeat as well as the NO9-15 repeat itself. An oligo A stretch was found between the distal flanking sequence and the NO9-15 repeat region.

Structural organization of transposable element mdg4 from Drosophila melanogaster and a nucleotide sequence of its long terminal repeats

A mobile dispersed genetic element, mdg4 , approximately 7.5 kilobases (kb) long has been cloned from D. melanogaster genome. Chromosomal bands have only few sites of mdg4 , but it always hybridizes to the chromocenter. The location of mdg4 varies among D. melanogaster strains. Blot hybridization shows that, in contrast to other mdg elements, mdg4 sequences are rather heterogeneous. Only few copies are full-length. A strong amplification of mdg4 has occurred during the in vitro cultivation of cells involving only one mdg4 variant. Long terminal repeats (LTRs) and flanking sequences have been sequenced in two cloned copies of transposable element mdg4 . In both cloned copies of mdg4 , LTRs have an identical nucleotide sequence 479 bp long. The mdg4 is flanked by four-base-pair direct repeats, short mismatched palindromes being present at the ends of each LTR. The termini of the mdg4 body contain an oligopurine stretch and a region partially complementary to D. melanogaster tRNA-Lys. Thus, structural organization of mdg4 LTRs is similar to that of several other mdg elements and retroviral proviruses.

Circular extrachromosomal copia-like transposable elements in Drosophila tissue culture cells

We find that in the circular extrachromosomal DNA from Drosophila tissue culture cells the transposable elements copia, 412, 297, and mdg 1 are present in variable amounts. There is no detectable circular DNA homologous to B104 . From the relationship between the intra- and extrachromosomal forms it appears that the amount of different circular elements is not related to the amount of the respective chromosomal elements.

The 5' termini of RNAs encoded by the transposable element copia

The 5' termini of copia-specific RNAs in Drosophila melanogaster tissue culture cells were determined by S1 nuclease mapping and cap analysis. Both major copia RNAs share an identical set of heterogeneous 5' ends. Three major cap 1 structures M7GpppCmpUp, M7GpppCmpCp and M7GpppGmpUp together with several other minor caps were found. Almost all the 5' termini, as judged by S-1 nuclease mapping, were located either in a pyrimidine-rich part of the terminal direct repeat or apparently outside of the copia element, suggesting that a proportion of copia transcripts derive from promoters external to the genetic element.

Ribosomal insertion-like elements in Drosophila melanogaster are interspersed with mobile sequences

Sequences homologous to the type 1 ribosomal DNA insertion occur in the chromocenter and in one location on the fourth chromosome of Drosophila melanogaster. Ribosomal insertion-like elements frequently occur in the form of tandem repeats and are often interspersed with nonhomologous DNA. Sequences that interrupt of flank insertion-like elements are members of diverged repeated DNA families. Members of five of these interspersed sequence families are located at multiple euchromatic sites as well as in the chromocenter. The euchromatic sites differ greatly in the gt-1 and gt-X11 stocks of D. melanogaster, suggesting that these sequence families are mobile in the genome. No long inverted repeats were detected in any of these interspersed sequences. One sequence, called 101F, interrupts a ribosomal insertion-like element; the nucleotide sequences across the boundaries between 101F and ribosomal insertion have been determined. A stretch of 13 base pairs that is present once in the ribosomal insertion is repeated at the ends of the 101F sequence, suggesting a target site duplication. Within the 101F element, no extended direct or inverted repeat sequence exists. The interspersed repeated sequences studied are transcribed into rare, heterogeneous, poly(A)-lacking nuclear RNA molecules. In one case, we showed that both strands of a flanking sequence are transcribed, but to a different extent. With respect to structural features and the nature of their transcripts, the sequences described here are distinct from other known transposable elements in Drosophila.

Mobile dispersed genetic element MDG1 of Drosophila melanogaster: nucleotide sequence of long terminal repeats

Long terminal repeats (LTRs) of two members of mdg1 family were sequenced. In the both cases, they are represented by perfect direct repeats 442 and 444 bp in length. Sixteen nucleotides in the LTRs of two different mdg1 elements are different. Each LTR contains slightly mismatched 16-nucleotide inverted repeats located at the ends of the LTR. Six base pairs closest to the termini of LTR form perfect inverted repeats. On the gene-distal sides of LTRs, short 4-nucleotide direct repeats are located, probably representing the duplication of a target DNA sequence arising from insertion of mdg. They are different in the two cases analyzed. Just as the other analyzed eukaryotic transposable elements, mdg1 starts with TGT and ends with ACA. Within the both strands of LTR, the sequences similar to Hogness box (a putative signal for RNA initiation, or a selector) and AATAAA blocks (putative polyadenylation signals) are present. The LTR of mdg1 contains many short direct and inverted repetitive sequences. These include a 10-nucleotide sequence forming a perfect direct repeat with the first ten nucleotides of the LTR. A region of LTR about 70 bp long is represented by simple repetitive sequences (TAT).

Chromosomal locations of two DNA segments that flank ribosomal insertion-like sequences in Drosophila: flanking sequences are mobile elements

We report the chromosomal locations of two repetitive DNA sequences that flank ribosomal insertion-like sequences in Drosophila melanogaster. The chromocentric region of D. melanogaster contains many copies of sequences that are homologous to type 1 ribosomal insertions. These insertion-like elements are interspersed with other DNA segments that we call flanking sequences. Two distinct flanking sequences derived from the same cloned DNA molecule pDmI 101, the HindIII fragments 101E and 101F, were studied. Whole genome Southern blots with DNA from the D. melanogaster stocks Oregon R (P2), gt-1, and gt-X11 showed complex restriction patterns that differed substantially between the three stocks. This and other data show that flanking sequences are members of diverged repetitive sequence families. In situ hybridization to salivary gland chromosomes of gt-1 and gt-X11 showed that both sequences are homologous to the chromocenter and to about 5 to 8 (101E) or 25 to 30(101F) euchromatic sites in each stock. Most, if not all, of these sites differed in gt-1 and gt-X11. Both 101E and 101F are homologous to he chromocenter and very few euchromatic bands in D. simulans, but 101F is homologous to numerous bands in D. mauritiana. We conclude that the flanking sequences represented by 101E and 101F are mobile elements within the genome of Drosophila. These two sequences differ in several structural features from mobile DNA elements previously described in this organism.

Mobile dispersed genetic element MDG1 of Drosophila melanogaster: transcription pattern

Virtually the whole region of the mobile dispersed genetic element mdg1 including the direct repeats that frame mdg1 /1/ is transcribed. The end of the transcriptional unit is located within the direct repeat. Although the transcription of mdg1 is symmetric, one direction is transcribed preferentially and is a full-length transcript represented by approximately 29S poly(A)+RNA. Besides it, approximately 15S poly(A)+RNA is present in the cytoplasm of culture cells which corresponds to the 3'-part of 29S RNA and possibly to a small region near the 5'-end of the latter. The major fraction of mdg1 transcripts is present in the fraction of free cytoplasmic RNP particles while only 10-20% are recovered in polysomes. In this respect, mdg1 is similar to mdg3 and quite different from two single-copy genes transcripts of which are almost completely recovered in polysomes. The relationship between mobile dispersed genetic elements and endogeneous viral genomes is discussed.