8.10 : Transkriptions-Elongationsfaktoren

Once transcription is initiated in the cell, the initial transcription factors are released from the pre-initiation complex. 

The RNA polymerase must then add new nucleotides to the 3′ end of the growing RNA strand in a phase called transcription elongation. 

Elongation in eukaryotes is challenging, as the DNA in a non-dividing cell exists as a condensed network called the chromatin. 

In the chromatin, the DNA is tightly wound around charged histone proteins at repeated intervals. These DNA–histone complexes are called nucleosomes.

When the RNA polymerase encounters the nucleosomes, or other DNA binding proteins, or some specific DNA sequences, it may halt. The inability to translocate further may lead to dissociation of the RNA polymerase before the complete gene is transcribed. 

To avoid this, the cell recruits special accessory proteins that can help the RNA polymerase execute an uninterrupted elongation process on the gene. 

The eukaryotic elongation factors associate directly with RNA polymerase and help it move smoothly along the template DNA strand and carry out its catalytic activity. 

In addition, the cell also recruits some other proteins, such as ATP-dependent chromatin remodelling complex and histone chaperones, which allow the transcription machinery to access the condensed genomic DNA inside the chromatin.

These multi-subunit complexes disrupt the interaction between the histone core and the DNA resulting in the restructuring or repositioning of the nucleosomes. 

Alteration in the nucleosome architecture helps create nucleosome-free regions of the DNA that can be easily accessed by the transcription machinery. 

Once the pre-mRNA is synthesized, the histones must be reinstated on the DNA template. The chromatin remodelling proteins and the histone chaperones thereafter, rewind the DNA around the histone proteins, completing the nucleosome reassembly process.