Cloning, genomic organization, and expression analysis of zebrafish nuclear receptor coactivator, TIF2

Thyroid hormone receptors (TRs) are involved in numerous diverse biological processes such as growth and differentiation, thermogenesis, neurulation, homeostasis, and metamorphosis. In zebrafish, TRbeta1 has been implicated to be involved in the obligatory embryonic-to-larval transitory phase. In order to understand if nuclear receptor coactivators could modulate the transcriptional activities of TRs during this transitory phase, the transcriptionary intermediary factor 2 (TIF2), a member of the p160 coactivator, was isolated from zebrafish. The zebrafish tif2 cDNA encodes a polypeptide of 1,505 amino acids. The tif2 gene is made up of 23 exons with the AUG and stop codon located in Exon IV and XXIII, respectively. The overall genomic organization of human and zebrafish tif2 genes are very similar. Four tif2 isoforms were identified by RT-PCR. The N-terminus mRNA variants are generated as a result of multiple initiation start sites located upstream of the noncoding Exon I and Exon II. The C-terminus isoforms, E20a and E20b, resulted from the alternative splicing of Exon XX. Although E20a and E20b isoforms were ubiquitously expressed, they were very highly expressed in reproductive tissues. The availability of TIF2 cDNA will allow the analysis of its functional roles in mediating the actions of TRs in various aspects of zebrafish developmental biology.