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Fig. 2. The Ubx PRE indiscriminately silences reporter genes that contain
an enhancer and the Ubx proximal promoter. (Left) The reporter genes
are very similar to those shown in Fig.
1 but the TATA box minimal promoter is replaced by a 4.1 kb
fragment that contains the proximal Ubx promoter and 5' UTR
(thin line, transcription start site marked by arrow). (Right) X-gal staining
of wing imaginal discs from transformant lines carrying the indicated reporter
gene construct with (left column) and without (right column) the PRE
fragment; in each case, the disc on the left is from the original transformant
line and the disc on the right is from its derivative line, obtained after
excision of the PRE in the germline. Note that in the presence of the
PRE fragment all three reporter genes are completely silenced. In the
derivative lines lacking PRE, the intensity of X-gal staining
suggests that the vgQE enhancer directs higher expression
levels from the Ubx promoter than from the TATA box promoter;
conversely, the dppWE enhancer activates expression more
potently from the TATA box promoter than from the Ubx promoter
(compare with Fig. 1).
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