Superunstable alleles at the cut locus in Drosophila melanogaster

Complete reversions of a gypsy retrotransposon-induced cut locus mutation in Drosophila melanogaster involving jockey transposon insertions and flanking gypsy sequence deletions

We have analysed the structures of three phenotypic revertant alleles of a gypsy retrotransposon-induced mutation at the cut locus of Drosophila melanogaster. All three revertants are associated with the insertion of jockey transposons into a common region of gypsy. Two of these alleles are complete reversions to wild type. One complete revertant (ct+D) is derived from a third allele, a partial revertant (ctMRpD) by a deletion of part of the gypsy sequence flanking the jockey transposon. Sequence differences between the jockey elements in ctMRpD and ct+D suggest that this deletion may have been created by the insertion of a second jockey near to the first, followed by recombinational excision of a composite jockey and the region between the two genetic elements. The other complete revertant also carries a deletion of gypsy DNA flanking the jockey insertion. The deleted regions of both complete revertants and the target region for all the jockey insertions contain a repeated sequence that resembles a transcriptional enhancer. The strength of the cut phenotype in these mutants correlates with the proportion of this region remaining near the gypsy transcriptional start site, suggesting that the jockey insertions relieve the gypsy-induced mutation at cut by interfering with a region which is required for the transcriptional competence of gypsy.

Mobile elements and transposition events in the cut locus of Drosophila melanogaster

We have cloned from the Oregon R strain of Drosophila melanogaster a 240 kb segment of DNA that contains the cut (ct) locus, and characterized the region for the presence of repetitive elements. Within this region at least five copies of the suffix element were detected, as well as several putatively novel mobile elements. A number of mutations obtained from the unstable ctMR2 strain and its derivatives were mapped within the cut locus. Comparison between parental and daughter strains indicates that frequently two or more independent transposition events involving the cut locus occur simultaneously within a single germ cell, thus providing a molecular basis for the transposition explosion phenomenon.

Mobile genetic elements in animal cells and their biological significance

Mobile genetic elements were discovered by McClintock while analysing unstable mutations in maize. The structural and functional studies of such elements became possible after their cloning, first from the genome of Drosophila melanogaster. In particular, Ilyin et al. demonstrated the varying location of the described elements in D. melanogaster chromosomes, thus providing the first evidence of their mobility. Mobile elements comprise a significant part of the genetic material in D. melanogaster (not less than 10%). Several classes of mobile elements do exist. Mobile dispersed genetic elements (mdg elements) are among the best characterized ones. Mdg elements are represented in the genome by dozens of families, each consisting of 10-150 copies. They are very similar structurally to proviruses of endogenous retroviruses. In particular, the both contain long terminal repeats (LTRs). The nucleotide sequences of LTRs and their flanking sequences of several mdg elements were determined. Their analysis suggested that RNA reverse transcription should be involved in the mdg amplification. It has been found that putative transposition intermediates, i.e. extrachromosomal DNA copies of mdg elements, are synthesized by reverse transcriptase in D. melanogaster culture cells. Another type of mobile genes is represented by P factor and similar elements. P factor seems to encode 'transposase' participating in direct excision and insertion of P elements themselves as well as of other mobile genes (mdg and fold-back elements). Besides these 'active transposons' which encode the enzyme machinery for transposition, a number of other sequences which may be transposed are present in the genome. RNAs synthesized on such elements can serve as a template for reverse transcriptase, and the DNA formed can then be inserted at new sites of the genome. Among such sequences are the so-called short ubiquitous repeats: B1 and B2 in mouse genome and Alu in human genome. We found that, at least in several cases, B-type sequences were located at the 3' end of mRNA. Short repetitive sequences were also detected at the 3' end of certain mRNAs of D. melanogaster. Usually the transpositions of mobile genes occur very rarely. However, under certain conditions, for example, in hybrid dysgenesis, they become more frequent. The strain with a mutation in cut locus was obtained in hybrid dysgenesis. This mutation depends on an insertion of mdg4 at the cut locus. Genetic instability in this strain is maintained for a long time. 'Transposition bursts' were found to occur in some germ cells.(ABSTRACT TRUNCATED AT 400 WORDS)