Molecules that start transcription

For a protein-coding gene to be expressed, the DNA is first transcribed onto a messenger RNA molecule. From there, it is translated into the amino acid sequence of the protein. This seemingly simple process is quite complex and scientists have made amazing progress in identifying a multitude of molecules and subunits that play important roles. In essence, binding and formation of molecular complexes on the DNA itself control and initiate transcription.

Upstream of the DNA sequence to be transcribed is a promoter region, the TATA box (so-called because it contains a specific sequence of thymine, T, and adenine, A). Binding of a specific protein there (the TATA box-binding protein that is part of transcription factor IID (TFIID)) initiates assembly of the pre-initiation complex (PIC) of which it is a part and which also contains RNA polymerase II (RNAPII). This initiates RNAPII transcription. Two more general transcription factors play a role, IIA and IIB (TFIIA and TFIIB, respectively), stabilising the PIC assembly.

To date, detailed structural information regarding DNA, TFIIA, TFIIB and TFIID is limited. EU-funded scientists working on the project PICENGINEERING created a DNA template for PIC assembly containing TFIID recognition elements, including TATA box. They cloned it and produced it in large-scale quantities for reconstitution of the PIC stage and detailed analysis of the complexes and their interactions.

The experimental research was quite challenging. Crystallisation studies aimed at determining the complex quaternary structure of a sub-complex containing all the relevant transcription factors (QUART) were laborious. However, they resulted in determination of the structure of TFIIA in its apo (inactive or isolated) form for the first time.

PICENGINEERING made important progress in the unravelling of structures and structural changes involved in the complex process of RNAPII transcription. With creation of the necessary DNA template including relevant sequences, the researchers are ready to embark on extensive crystallisation trials to study the supramolecular architecture of the PIC stage. Studies are expected to provide insight into the long-sought 3D structure of QUART and reveal the network of interactions involved in RNAPII transcription initiation.