LTR RETROTRANSPOSONS AND THE EVOLUTION OF EUKARYOTIC ENHANCERS

Since LTR retrotransposons and retroviruses are especially prone to re gional duplications and recombination events, these viral-like systems may be especially conducive to the evolution of closely spaced combin atorial regulatory motifs. Using the Drosophila copia LTR retrotranspo son as a model, we show that a regulatory region contained within the element's untranslated leader region (ULR) consists of multiple copies of an 8 bp motif (TTGTGAAA) with similarity to the core sequence of t he SV40 enhancer. Naturally occurring variation in the number of these motifs is correlated with the enhancer strength of the ULR. Our resul ts indicate that inter-element selection may favor the evolution of mo re active enhancers within permissive genetic backgrounds. We propose that LTR retroelements and perhaps other retrotransposons constitute d rive mechanisms for the evolution of eukaryotic enhancers which can be subsequently distributed throughout host genomes to play a role in re gulatory evolution.