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An RNA Transposition Intermediate
With the similarity of the yeast Ty1 sequence to retroviruses, it was of considerable interest to determine whether the element transposes using an RNA intermediate or a DNA intermediate. One simple way to test whether Ty1 proceeds through an RNA intermediate was to make use of the fact that in eukaryotic cells intervening sequences are re-moved from mRNA. Fink and co-workers placed a strong and control-lable promoter upstream of a Ty element (Fig. 19.18). In the middle of the element they placed a segment of a yeast ribosomal protein gene containing an intervening sequence. Inducing the promoter led to a high frequency of transposition. Most importantly, the transposed elements lacked the intervening sequence, thereby proving they had transposed
Figure 19.18 When Ty1 passes through a mRNA state, the intervening se-quence in the ribosomal protein gene is spliced out and the resulting transposon, which drives His expression, lacks the sequence.
via an RNA intermediate, and while in the RNA state, had been subjected to the cellular processing that removes intervening sequences.
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