Identification of an unusual structure in the Drosophila melanogaster transposable element copia: evidence for copia transposition through an RNA intermediate

TAHRE, a novel telomeric retrotransposon from Drosophila melanogaster, reveals the origin of Drosophila telomeres

Drosophila telomeres do not have typical telomerase repeats. Instead, two families of non-LTR retrotransposons, HeT-A and TART, maintain telomere length by occasional transposition to the chromosome ends. Despite the work on Drosophila telomeres, its evolutionary origin remains controversial. Herein we describe a novel telomere-specific retroelement that we name TAHRE (Telomere-Associated and HeT-A-Related Element). The structure of the three telomere-specific elements indicates a common ancestor. These results suggest that preexisting transposable elements were recruited to perform the cellular function of telomere maintenance. A recruitment similar to that of a retrotransposal reverse transcriptase has been suggested as the common origin of telomerases.

Invertebrate retroviruses: ZAM a new candidate in D.melanogaster

ZAM, a new retroelement of Drosophila melanogaster, was identified as a mutational insertion at the white locus. It displays all the structural features of a vertebrate retrovirus. Its three open reading frames encode predicted products resembling the products of the gag, pol and env genes of retroviruses. Its transcription gives rise to an 8.6 kb full-length RNA and a 1.7 kb spliced message for the env gene. The latter encodes an envelope protein that is typical of elements having an extracellular phase of the life cycle. The identification of a ZAM envelope retrogene provides evidence that ZAM is mobilized through a reverse trancriptional process in the germ line of flies. We report that ZAM is distributed differently among D.melanogaster strains. Two stocks out of >15 tested display a ZAM high copy number, with numerous copies distributed on chromosomal arms. This high copy number is associated with a high transcriptional rate of ZAM. The existence of these two categories of strains offers a new genetic system in which the properties of a potential invertebrate retrovirus can be tested.

Characterization and cloning of p11, a transrepressor of Drosophila melanogaster retrotransposon 1731

The NssBF element has been characterized as a 26 nt sequence in the long terminal repeat of Drosophila melanogaster retrotransposon 1731. This sequence has been shown to be implicated in transcriptional repression of the 1731 promoter. We here report the cloning of a cDNA encoding a nuclear DNA binding protein named p11 that binds specifically to the NssBF element. P11 is a 98 amino acid polypeptide. It exhibits similarities with the mouse p9 single-stranded DNA binding protein, raising the possibility of a very general family of protein factors. Co-transfection experiments in human U937 cells showed repression of the 1731 promoter by overexpression of p11.

Zeon-1, a member of a new maize retrotransposon family

We have previously shown that the tandemly duplicated 27 kDa maize storage protein locus underwent mitotic rearrangement to yield a single-copy allele in isolates of the inbred line A188. This rearrangement contains a new LTR retrotransposon, designated Zeon-1. This middle repetitive element of 7313 bp had two long terminal repeats, a primer binding site, a polypurine tract and a gag-related open reading frame of 375 amino acids. Transcripts of the gag-related region were detected by the polymerase chain reaction (PCR) in certain maize tissues, and Western blots detected the gag-related protein in the same tissues. Moreover, the product of this mitotic rearrangement was shown to contain the same insertion site and 3' LTR as Zeon-1, suggesting that this rearrangement occurs with unusual precision. Zeon elements were found to be present in teosinte and not present in the Gramineae wheat, barley, sorghum and rye.

Spliced HERV-H endogenous retroviral sequences in human genomic DNA: evidence for amplification via retrotransposition

HERV-H elements are a large family of endogenous retrovirus-like sequences found in approximately 1000 dispersed copies in the genomes of humans and other primates. The most abundant subclass of these elements is a partially deleted form of 5.8 kb which is transcribed primarily as a 5.6-kb unit length RNA and a 3.7-kb spliced derivative. The provirus-like structure of these elements suggests that their numbers have increased in the genome through retrotransposition. However, this has not been demonstrated for HERV-H. To determine if genomic expansion of HERV-H elements involved an RNA intermediate, primate DNAs were screened by PCR for elements that were transcribed, spliced, reverse transcribed, and integrated back into the genome. This PCR screen detected several genomic HERV-H fragments that appear to be derived from spliced transcripts. Interestingly, the presence of one of these fragments is polymorphic in humans, suggesting that its integration was a relatively recent event. Another PCR strategy was used to determine that at least one of the spliced elements has an intact 5' LTR, indicating that it is not simply a "processed pseudogene" or cDNA copy of a HERV-H transcript. Genomic cloning and sequencing of a human locus harboring a spliced element revealed the expected structure, e.g., intact LTRs and flanking 5-bp direct repeats, for a virally retrotransposed element. A genomic library screening method also indicated that very few HERV-H elements (less than 1%) have the structure of processed pseudogenes. These results suggest that most HERV-H elements amplified in the genome as viral retrotransposons.

The NssBF element, a sequence of the Drosophila melanogaster retrotransposon 1731 potentially implicated in transcriptional repression and replication

The nuclear single-stranded DNA binding factor (NssBF) has been characterized as a nuclear protein that binds to a 26 nucleotides sequence in the long terminal repeat (LTR) of the Drosophila melanogaster 1731 retrotransposon. This sequence, called NssBF element, was analysed by gel retardation experiments using wild-type and mutated oligonucleotides. In vitro transcription experiments were performed and suggest that NssBF element binding protein(s) represses transcription through the 1731 promoter. Furthermore, computer assisted sequence comparisons put forward a possible role of this element and/or its associated DNA binding proteins in replication.

Efficient amplification of Drosophila simulans copia directed by high-level reverse transcriptase activity associated with copia virus-like particles

The number of retrotransposon copia per genome in Drosophila melanogaster cultured cells is two to three times higher than that in D. melanogaster embryo cells. Here, we have found that the genome of the related species, Drosophila simulans, contains in cultured cells more efficiently amplified copia DNA (approximately ten fold). Furthermore, we analyzed copia virus-like particles (VLPs) prepared from D. melanogaster and D. simulans cultured cells, which contain copia RNA and reverse transcriptase (RT) activity, and thus, play a major role in copia replication. The RT activity associated with the D. simulans VLPs was 25 times higher than that associated with the D. melanogaster VLPs. Taken together with the fact that copia is believed to transpose through an RNA intermediate, these results suggest that the amplification of copia DNA should relate to copia RNA-mediated transposition, and the higher RT activity associated with the D. simulans VLPs would lead to the efficient amplification of copia DNA. In a comparison between D. melanogaster and D. simulans copia nucleotide (nt) sequences, five nt substitutions, which cause the respective amino acid changes, were found in the copia RT-coding region. Polymerase chain reaction direct sequencing showed that these five substitutions are the vast majority in each Drosophila species. The substitutions, therefore, may be responsible for the high level of the RT activity associated with the D. simulans VLPs.

Production of a unique multi-lamella structure in the nuclei of yeast expressing Drosophila copia gag precursor

Drosophila retrotransposon copia produces virus-like particles (VLPs) in the nuclei of cultured Drosophila cells. The VLPs contain copia RNA and reverse transcriptase activity, and thus, play a major role in copia replication. Here we have expressed the copia gag polyprotein precursor in yeast. The precursor, which includes copia protease itself, showed correct autoprocessing to produce a unique multi-lamella structure in the nuclei of the yeast cells. This expression system should be useful for the analysis of nuclear localization of the major copia VLP protein, and furthermore, would provide important information concerning the mechanism of copia VLPs formation.