Qualitative and quantitative analyses of the transpositions of <Emphasis Type="Italic">P</Emphasis> element—based genetic construction into the region of <Emphasis Type="Italic">Drosophila melanogaster hsp70</Emphasis> genes

The hsp70 genes is among the main systems underlying the adaptation of organisms to adverse environmental factors. The ever increasing amount of data in literature demonstrates an important adaptive role of mobile genetic elements in microevolution. Drosophila hsp70 genes are potential target for transpositions of various mobile elements in natural populations. We have analyzed the frequency and localization of a P element—based genetic construction, EPgy2, in the region of Drosophila melanogaster hsp70 genes. A hot spot for the transposition was discovered in the promoter regions of genes hsp70Aa and hsp70Ab. No insertions of this construction in the coding or 3′-flanking regions of hsp70 genes have been recorded. It was demonstrated that the region of 161 to 7800 bp adjacent to the original construction is in certain cases also involved in the transposition. No transpositions of any other mobile elements have been observed. The inserts were shown to change the activity of hsp70 genes and the thermotolerance of transgenic strains.     

Induction of single transpositions of mobile genetic elements in Drosophila melanogaster using mitomycin C

Intraperitoneal injections of mitomycin C into the males from laboratory strains of Drosophila melanogaster induce several mutation events in different loci of the X-chromosome in the offspring. These mutations are caused by transposition of mobile genetic elements. The transpositions are single and are not associated with transposition explosions.     

Drosophila melanogaster is polymorphic for a specific repeated (CATA) sequence in the regulatory region of hsp23.

To identify sequence variation associated with a selection response for heat tolerance in Drosophila melanogaster, we sequenced 1400bp of the heat shock protein 23 gene (hsp23) promoter region in four heat-selected and two control lines. The region was found to be variable for a specific (CATA) repeated sequence, and the sequence CTT seems to be a hot spot for mutation. The repeated tetranucleotide sequence was located in several short repeats scattered throughout the entire region. Similar variable repeats are also located downstream the of hsp23 gene in the intergenic region between hsp23 and hsp27. We detected nine different hsp23 alleles. Their frequencies in the selection and control lines seemed to be mainly determined by genetic drift. The function of the CATA repeats is not yet known, though these regions have homology to SAR elements located in the intergenic region between two hsp70 genes, suggesting a similar function.     

Duplicative and conservative transpositions of larval serum protein 1 genes in the genus Drosophila

Interspecific comparative molecular analyses of transposed genes and their flanking regions can help to elucidate the time, direction, and mechanism of gene transposition. In the Drosophila melanogaster genome, three Larval serum protein 1 (Lsp1) genes (alpha, beta and gamma) are present and each of them is located on a different chromosome, suggesting multiple transposition events. We have characterized the molecular organization of Lsp1 genes in D. buzzatii, a species of the Drosophila subgenus and in D. pseudoobscura, a species of the Sophophora subgenus. Our results show that only two Lsp1 genes (beta and gamma) exist in these two species. The same chromosomal localization and genomic organization, different from that of D. melanogaster, is found in both species for the Lsp1beta and Lsp1gamma genes. Overall, at least two duplicative and two conservative transpositions are necessary to explain the present chromosomal distribution of Lsp1 genes in the three Drosophila species. Clear evidence for implication of snRNA genes in the transposition of Lsp1beta in Drosophila has been found. We suggest that an ectopic exchange between highly similar snRNA sequences was responsible for the transposition of this gene. We have also identified the putative cis-acting regulatory regions of these genes, which seemingly transposed along with the coding sequences.     

Low occurrence of gene transposition events during the evolution of the genus Drosophila

Abstract.— The role played by gene transpositions during the evolution of eukaryotic genomes is still poorly understood and indeed has been analyzed in detail only in nematodes. In Drosophila , a limited number of transpositions have been detected by comparing the chromosomal location of genes between different species. The relative importance of gene transposition versus other types of chromosomal rearrangements, for example, inversions, has not yet been evaluated. Here, we use physical mapping to perform an extensive search for long-distance gene transpositions and assess their impact during the evolution of the Drosophila genome. We compare the relative order of 297 molecular markers that cover 60% of the euchromatic fraction of the genome between two related Drosophila species and conclude that the frequency of gene transpositions is very low, namely one order of magnitude lower than that of nematodes. In addition, gene transpositions seem to be events almost exclusively associated with genes of repetitive nature such as the Histone gene complex ( HIS-C ).     

Transposition of mobile dispersed genes (MDG) after substitution of various chromosome pairs in inbred Drosophila melanogaster strains

Transpositions of MDG-1, MDG-3 and copia were detected as a result of crosses of the inbred maladaptive LA stock with laboratory stocks, in order to construct the genomes carrying different combinations of the LA or non-La chromosomal pairs. Changes of the mobile gene distributions were revealed in chromosomes of hybrid genotypes, as compared to parental chromosomal pairs. A trivial source of variability of chromosomal molecular structure ensured by crossing over was excluded by inversions which serve as suppressors of crossing over in corresponding crosses. Multiple transpositions of mobile genes in definite chromosomal sites were detected in genotypes carrying chromosomal pair 2 originated from the LA stock. No such transpositions were observed, when the pair 2 was substituted by the chromosome 2 originated from the Swedish-b line or in control crosses, where the LA stock was not involved. Both LA chromosomes 2 and 3 were shown to be the targets of transpositions. Comparison of hot spot transposition sites of MDG-1, as a result of crosses, with the earlier described rare events of spontaneous transpositions in the LA stock, coupled with its fitness increase, revealed that the hot spot sites were shared in both series of experiments. The data obtained show that transpositions of mobile genetic elements may change the genetic and molecular structure of the chromosome involved in crosses, in spite of suppression of crossing over by inversions usually suggested as a tool for keeping chromosomal genetic structure intact.     

ISCR elements: novel gene-capturing systems of the 21st century?

"Common regions" (CRs), such as Orf513, are being increasingly linked to mega-antibiotic-resistant regions. While their overall nucleotide sequences show little identity to other mobile elements, amino acid alignments indicate that they possess the key motifs of IS91-like elements, which have been linked to the mobility ent plasmids in pathogenic Escherichia coli. Further inspection reveals that they possess an IS91-like origin of replication and termination sites (terIS), and therefore CRs probably transpose via a rolling-circle replication mechanism. Accordingly, in this review we have renamed CRs as ISCRs to give a more accurate reflection of their functional properties. The genetic context surrounding ISCRs indicates that they can procure 5' sequences via misreading of the cognate terIS, i.e., "unchecked transposition." Clinically, the most worrying aspect of ISCRs is that they are increasingly being linked with more potent examples of resistance, i.e., metallo-beta-lactamases in Pseudomonas aeruginosa and co-trimoxazole resistance in Stenotrophomonas maltophilia. Furthermore, if ISCR elements do move via "unchecked RC transposition," as has been speculated for ISCR1, then this mechanism provides antibiotic resistance genes with a highly mobile genetic vehicle that could greatly exceed the effects of previously reported mobile genetic mechanisms. It has been hypothesized that bacteria will surprise us by extending their "genetic construction kit" to procure and evince additional DNA and, therefore, antibiotic resistance genes. It appears that ISCR elements have now firmly established themselves within that regimen.     

Induction of unstable mutations in Drosophila melanogaster by microinjection of oncogenic virus DNA into the embryo polar plasma. Insertional nature of mutations]

We have demonstrated that mutations induced in Drosophila melanogaster by the microinjections of adenovirus Sa7 DNA in early embryos are of insertional nature. The role of insertional elements is played by the Drosophila transposons, but not by the virus DNA. The ability of oncoviral DNA to induce transpositions of mobile elements in recipient genome is the molecular basis of this system of genetic instability.     

The role of a positioned nucleosome at the Drosophila melanogaster hsp26 promoter.

The regulatory region of Drosophila melanogaster hsp26 includes a positioned nucleosome located between the two DNase I hypersensitive (DH) sites that encompass the critical heat shock elements (HSEs). To test the role of this nucleosome in regulated expression, transgenic flies containing hsp26-lacZ fusion genes with alterations in the nucleosome-associated region have been generated. The positioned nucleosome is associated with a DNA sequence that does not itself contain any critical regulatory elements for heat shock-inducible expression. The nucleosome-associated sequence can be deleted, reversed, duplicated or replaced by a random sequence with no significant effect on DH site formation and gene expression. Analyses of hsp26 and hsp70 transgenes with spacing changes within the promoter region indicate that the location of the (CT)n.(GA)n elements dictates the location of DH site formation. Wrapping the DNA between the regulatory elements around a nucleosome is as effective for gene expression as placing the regulatory elements close to each other. A loss of inducible gene expression was observed when the nucleosome-associated DNA was replaced with sequences which appear to misdirect nucleosome placement. The results indicate considerable flexibility in the spacing between DH regulatory sites.     

The nature of spontaneous and induced mutagenesis in a genetically unstable mutator strain of Drosophila melanogaster]

The spontaneous and induced frequencies of visible mutations by N-nitroso-N-ethylurea in male cells of Drosophila melanogaster genetically unstable mutator strain have been investigated. The spontaneous and induced by N-nitroso-N-ethylurea genetic instability in mutator strain have similar manifestation, that evidently testifies the existence of general mechanisms of the appearance of unstable mutations, namely the transpositions of the mobile genetic elements.     

Analysis of Subtelomeric Heterochromatin in the Drosophila Minichromosome Dp1187 by Single P Element Insertional Mutagenesis

We investigated whether single P element insertional mutagenesis could be used to analyze heterochromatin within the Drosophila minichromosome Dp1187. Forty-five insertions of the P[lacZ,rosy+] element onto Dp1187 (recovered among 7,825 transpositions) were highly clustered. None was recovered in centromeric heterochromatin, but 39 occurred about 40 kb from the distal telomere within a 4.7-kb hotspot containing tandem copies of a novel 1.8-kb repetitive DNA sequence. The DNA within and distal to this region lacked essential genes and displayed several other properties characteristic of heterochromatin. The rosy+ genes within the inserted transposons were inhibited by position-effect variegation, and the subtelomeric region was underrepresented in polytene salivary gland cells. These experiments demonstrated that P elements preferentially transpose into a small subset of heterochromatic sites, providing a versatile method for studying the structure and function of these chromosome regions. This approach revealed that a Drosophila chromosome contains a large region of subtelomeric heterochromatin with specific structural and genetic properties.