446 Chapter 13 • The Dynamic Genome: Transposable Elements unknown mechanism, triggered the suppression of both itself and the homologous gene in the petunia chromo-some. This phenomenon has come to be known as cosuppression. As the transformation of some plant species with foreign genes became routine, scientists noticed that a variety of transgenes were efFciently silenced in the plant host genome. Because the transgenes could often be reactivated, this silencing was recognized as a form of epigenetic regulation. However, it is highly unlikely that organisms have evolved mechanisms to turn off trans-genes introduced by plant scientists. Instead, it was reasoned that transgenes are silenced because they re-semble a natural threat to the host, perhaps their own transposable elements or infecting viruses or both. Like transposable elements and viruses, transgenes can insert into new sites in the host genome. What if organisms had defense mechanisms that could recognize these “in-vaders” and turn them off by silencing their expression, possibly through changes in chromatin structure? In an attempt to identify host genes contributing to transgene and possibly transposable-element silencing, geneticists sought suppressor strains that had lost the ability to silence transgenes. One approach used a strain
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