Fig. 1.

Developmental expression profiles and T₃ regulation of TRβ, TH/bZIP, and IFABP in the intestine during natural and T₃-induced metamorphosis of X. tropicalis. A and B, The expression of two known direct TR target genes TRβ and TH/bZIP, and an indirectly repressed gene IFABP were analyzed during natural (A) and T₃-induced (B) metamorphosis by qRT-PCR. EF1α was also analyzed as the control gene (nonregulated) and used to normalize the expression of the other genes. Note that the levels of TRβ and TH/bZIP mRNA were gradually up-regulated during natural development and reached a peak at stage 60 and 62, respectively (A). The expression of mRNA was then reduced by stage 66, the end of metamorphosis (A). On the other hand, the level of IFABP mRNA was down-regulated at stage 62, at the climax of metamorphosis. After 2 d of T₃ treatment of premetamorphic tadpoles at stage 54, the expression of TRβ and TH/bZIP mRNA were up-regulated, whereas that of IFABP was down-regulated (B), just like during natural metamorphosis. Error bars indicate sem. C, Schematics of the promoters of X. tropicalis TRβ and TH/bZIP genes showing the locations of the TRE relative to the transcription start site (+1). Note that the positions were based on those for the corresponding X. laevis genes (52, 68) because the exact start sites of the X. tropicalis genes have not been mapped but the TRE regions are conserved (7). The arrows indicate the relative locations of the PCR primers used for the analysis of the ChIP DNA.