Thyroid hormone (T3) receptors (TRs) mediate T3 effects on vertebrate development. We have studied Xenopus tropicalis metamorphosis as a model for postembryonic human development and demonstrated that TRα knockout induces precocious hind limb development. To reveal the molecular pathways regulated by TRα during limb development, we performed chromatin immunoprecipitation- and RNA-sequencing on the hind limb of premetamorphic wild type and TRα knockout tadpoles, and identified over 700 TR-bound genes upregulated by T3 treatment in wild type but not TRα knockout tadpoles. Interestingly, most of these genes were expressed at higher levels in the hind limb of premetamorphic TRα knockout tadpoles than stage-matched wild-type tadpoles, suggesting their derepression upon TRα knockout. Bioinformatic analyses revealed that these genes were highly enriched with cell cycle and Wingless/Integrated (Wnt) signaling-related genes. Furthermore, cell cycle and Wnt signaling pathways were also highly enriched among genes bound by TR in wild type but not TRα knockout hind limb. These findings suggest that direct binding of TRα to target genes related to cell cycle and Wnt pathways is important for limb development: first preventing precocious hind limb formation by repressing these pathways as unliganded TR before metamorphosis and later promoting hind limb development during metamorphosis by mediating T3 activation of these pathways.

The histone acetyltransferase (HAT) subunit of coactivator complex SAGA, Gcn5, stimulates eviction of promoter nucleosomes at certain highly expressed yeast genes, including those activated by transcription factor Gcn4 in amino acid-deprived cells; however, the importance of other HAT complexes in this process was poorly understood. Analyzing mutations that disrupt the integrity or activity of HAT complexes NuA4 or NuA3, or HAT Rtt109, revealed that only NuA4 acts on par with Gcn5, and functions additively, in evicting and repositioning promoter nucleosomes and stimulating transcription of starvation-induced genes. NuA4 is generally more important than Gcn5, however, in promoter nucleosome eviction, TBP recruitment, and transcription at most other genes expressed constitutively. NuA4 also predominates over Gcn5 in stimulating TBP recruitment and transcription of genes categorized as principally dependent on the cofactor TFIID versus SAGA, except for the most highly expressed subset including ribosomal protein genes, where Gcn5 contributes strongly to PIC assembly and transcription. Both SAGA and NuA4 are recruited to promoter regions of starvation-induced genes in a manner that might be feedback controlled by their HAT activities. Our findings reveal an intricate interplay between these two HATs in nucleosome eviction, PIC assembly, and transcription that differs between the starvation-induced and basal transcriptomes.