TFIIH trafficking and its nuclear assembly during early Drosophila embryo development.

We present the first analysis of the dynamics of the transcription DNA-repair factor TFIIH at the onset of transcription in early Drosophila development. TFIIH is composed of ten polypeptides that are part of two complexes - the core and the CAK. We found that the TFIIH core is initially located in the cytoplasm of syncytial blastoderm embryos, and that after mitotic division ten and until the cellular blastoderm stage, the core moves from the cytoplasm to the nucleus. By contrast, the CAK complex is mostly cytoplasmic during cellularization and during gastrulation. However, both components are positioned at promoters of genes that are activated at transcription onset. Later in development, the CAK complex becomes mostly nuclear and co-localizes in most chromosomal regions with the TFIIH core, but not in all sites, suggesting that the CAK complex could have a TFIIH-independent role in transcription of some loci. We also demonstrate that even though the CAK and the core coexist in the early embryo cytoplasm, they do not interact until they are in the nucleus and suggest that the complete assembly of the ten subunits of TFIIH occurs in the nucleus at the mid-blastula transition. In addition, we present evidence that suggests that DNA helicase subunits XPB and XPD are assembled in the core when they are transported into the nucleus and are required for the onset of transcription.