Homologues of Drosophila P-transposons in vertebrate genomes

A substantial fraction of vertebrate and invertebrate genomes is composed of mobile elements and their derivatives. At least 45% of the human genome belongs to this type of DNA. One of the most intensively studied transposons are the P elements of Drosophila, which have been discovered in Drosophila melanogaster. Successively they were found to be widely distributed among Diptera, especially in the family Drosophilidae. Complete P elements are about 3 kb long and consist of four exons, three introns and non coding terminal regions flanked by terminal inverted repeats. For mobility they need the P element encoded transposase and additional host encoded protein factors. P elements were thought to exist exclusively in the genomes of dipteran species. Based on the data provided by the human genome project we recently identified a P element homologous sequence in the human genome. The P element homologous human gene, named Phsa, is 12445 bp long, consists of three exons and two introns, and encodes a protein of unknown function with a length of 759 aa. Because of the absence of terminal inverted repeats and the length of the gene, which is unusual for a DNA transposon, we suppose that Phsa is a stationary sequence. The widespread occurrence of P homologous sequences within vertebrate genomes is indicated by the detection of sequences homologous to Phsa and to the insect P elements in the genomes of the cow, the mouse and the chicken. In the course of the planned project structur and function of P-homologous sequences in different vertebrates (human, primates, cow, horse, dog, chicken, snake, frog, fish) shall be analyzed by molecularbiological and cellbiological techniques. The P-homologues will be isolated by PCR and sequenced. RT-PCR, 5`race and 3`race will be used to construct the corresponding cDNAs in order to determine the ends and the splicing patterns of the mRNAs. Transcription and expression patterns will be analyzed within the mouse system by whole-mount in-situ- hybridization and immunohisto-chemical localization using sections from different developmental stages of the mouse embryo. The results of the project will contribute to our understanding of the meaning transposable elements have for genome evolution.

The P elements of Drosophila belong to the class of DNA transposons such pertaining to the so-called "jumping genes" and were identified about 25 years ago as the causative agent of a syndrome called "hybrid dysgenesis" reflecting the transpositionally activity of them. 2001 our group identified P-homologous sequences in vertebrate genomes, refuting the 25 years lasting theory that P-elements occur exclusively in diptera. This finding was the basis of the project that aimed to bring more light into the evolution and function of P-homologous sequences within the vertebrate system. Complex P-homologous sequences could be detected in nearly all vertebrates. The sequence divergences of the analyzed P-homologous sequences are more or less in accordance with the phylogeny of their host species illustrating their vertical transmission. In contrast, horizontal transmission between sexually isolated taxa seems to play a major role in the life cycle of P-elements within the drosophilids. The single copy P-homologous sequences of human (Phsa) and chicken (Pgga) are lacking transposon characteristic features such as terminal inverted repeats (TIRs) and target site duplications (TSDs) and probably are coding for a functional protein with a N-terminal DNA-binding domain (THAP-domain). Phsa is expressed in a broad range of human tissues proving that the putative protein plays an essential but not yet understood role in a specific metabolic pathway. Another scenario could be analyzed for the zebrafish genome, where several P-homologous elements (Pdre-elements) with TIRs and TSDs exist. To explain the sequence arrangements of them the following evolutionary model is presumed: Their precursor sequence was created by integration of a P element-homologous gene into a transposon belonging to family referred to as demio elements either by insertion or by recombination thus capturing the transposon typical TIRs. To explain the composition of the probably only functional coding copy a further step has to be postulated during which the P-homologous gene, flanked by the demio-like border regions, was inserted into a further DNA transposon belonging to a family of DNA-transposons referred to as internally deleted Pdre elements. The "domestication hypothesis" considers the stationary transposon homologous sequences as degenerated transposons whereas the "source gene hypothesis" argues for an evolutionary scenario in which active P transposons are direct descendants of an ancient source gene. Our results suggest, that both scenarios are possible: The active progenitor sequence of the zebrafish P-elements may have evolved following the "source gene hypothesis". At least two of their recent descendents are immobile now. Since immobilization can be considered as requirement for a domestication event the data obtained from the zebrafish support the "domestication hypothesis" as well. Phsa and Pgga are located at orthologous positions, thus favoring the "source gene hypothesis", but as well they can be interpreted as descendents of a former active transposon which has been domesticated within a common ancestor before the separation of mammalians and birds about 310 MY ago.