P-repressor like proteins in Drosophila: functional studies on a domesticated TE-derived neogene-cluster

The genomes of the closely related species Drosophila guanche, D. madeirensis and D. subobscura contain tandemly amplified derivatives of once active P elements. Over evolutionary time, each 3.2 kb cluster-unit has selectively conserved the first three exons coding for a 65 kDa protein homologous to the functional P element repressor of D. melanogaster. All P-"repressor-like" proteins within the gene cluster analysed so far have conserved the two domains known to be of functional significance for canonical P-repressor activity, i.e. the N-terminal zinc finger-like motif and the coiled coil domain. In their upstream transcriptional regulatory section each unit harbours typical eukaryotic motifs of transcriptional initiation derived from insertions of other transposable elements (TEs). The cluster units are expressed in a stage and tissue specific pattern conserved in all three species. The complete lack of full-sized P elements in their genomes - the obvious targets for a functional P element repressor protein, as well as hypervariability within their potential N-terminal DNA-binding regions suggest that the P-"repressor-like" coding genes are in the progress of evolving into novel functional chromosomal genes - a process we designated as molecular domestication. By using genetic assays we have recently started to survey the functional abilities of the P-"repressor-like" protein of D. guanche in transgenic lines of D. melanogaster. These in vivo assays clearly demonstrated that the P-related protein lacks completely any canonical P-repressor activity in this system. Surprisingly, ectopic expression of the P-"repressor-like" protein in D. melanogaster reveals a significant enhancement-effect on P element mobility - a result which seems paradoxical for a P-repressor-related protein originated in a host devoid of active P elements. In the course of the three year project presented here we propose 1.) to survey the exact temporal and spatial expression pattern of variant transcription-units within the gene cluster which are regulated by novel promoters originated from nested insertions of other TEs and 2.) to extend our in vivo assays into the functional properties of P-"repressor-like" proteins and their interaction with the host genome.

Mobile DNAs or transposons present a class of genetic parasites that are capable of spreading throughout host genomes, moving their position within genomes and thus are regarded as potential mutagenic agents. They are universal components of all eukaryotic genomes and in some plants they can even comprise up to 90% of the genome. In humans at least half of our genome has been derived from derivatives of formally active mobile DNAs. Transposon-encoded proteins, essential for the successful genome-wide propagation of the genomic parasite harbor a very attractive repertoire of functional abilities for a cell. In the course of their coexistence with the host genome some transposon-encoded genes got domesticated, i.e. these sequences were coopted by the host by serving novel host-encoded functions. This functional transition of a formally genetic parasite took place repeatedly in the evolution of eukaryotic organisms. In the course of the Human Genome Sequencing Program at least 47 human host genes were recovered as probably derived from the coding sections of former active transposons. To date nothing is known about their evolutionary dynamics as well as their functional abilities in human and related species Therefore the P-transposon-derived neogene cluster of Drosophila studied here provides an excellent model system in order to understand the impact of mobile DNAs on the formation of new genes. In the course of this project we have elucidated that transposons of distinctive origin are capable of acting in a modular manner in generating novel host-encoded genes. Furthermore we have discovered that in the case of our model system the transposon-derived P-protein is capable of serving global regulatory epigenetic functions to the host.