Trans-silencing of euchromatic transgenes by a telomeric homologous transgene. S. Ronsseray , B. Philippini , A. Boivin , D. Nouaud , D. Anxolabehere. Dynamique du Genome et Evol, Inst Jacques Monod-CNRS, Paris, France.

   Autonomous P elements, located at the X chromosome telomere (1A site), have strong P regulatory properties that include the repression of both P-induced dysgenic sterility (GD sterility) and P-lacZ expression in the germline. These P elements are flanked by heterochromatic Telomeric Associated Sequences (TAS). P elements at 1A, as tested in vivo by accessibility to the Dam methylase from E.coli (introduced by transgenesis), present a reduced accessibility, suggesting that they are themselves heterochromatinized. P-lacZ insertions at 1A in TAS do not repress GD sterility; however, Roche and Rio have shown that they are able to repress in trans a euchromatic P-vasa-lacZ. This indicates that a cross-talk exists between telomeric and euchromatic transgene copies. Finally, we have isolated a new naturally occurring P element at 1A, deleted in 5'. It has a capacity to repress a euchromatic P-lacZ which depends on the homology between its structure and that of the transgene. Theses results suggest that the cross-talk involves a homology-dependent component. We are testing whether this trans-silencing is restricted to P sequences. We present experiments designed to test whether a euchromatic Hobo-lacZ can be repressed by a telomeric P-lacZ, because of the homology linked to the presence of the lacZ sequence. We are also testing whether the repressed state of a euchromatic P-lacZ, induced by a telomeric P-lacZ, is maintained in the next generation when the telomeric transgene is lost by segregation.