Transcription Elongation Factors

When RNA polymerase encounters nucleosomes or other DNA-binding proteins, it may halt and potentially dissociate before completing gene transcription. To prevent this, cells recruit accessory proteins including ATP-dependent chromatin remodeling complexes and histone chaperones that restructure nucleosomes, creating nucleosome-free regions for uninterrupted transcription.

Eukaryotic elongation factors associate directly with RNA polymerase to help it move smoothly along the template DNA strand and maintain catalytic activity. These factors enable the polymerase to overcome obstacles like chromatin condensation and specific DNA sequences that would otherwise interrupt the elongation process.

Reversible pauses occur when RNA polymerase temporarily halts, and proteins like TFIIF and elongins allow it to resume elongation. Irreversible pauses become transcriptional arrests where the polymerase cannot resume on its own. Elongation factors such as TFIIS and pTEFb enable RNA polymerase II to read through arrest sites and continue transcription.

In eukaryotes, transcription is coupled with mRNA processing, making pausing necessary around exon-intron junctions to increase mRNA splicing efficiency. These regulated halts allow the cell to coordinate transcription with alternative RNA splicing and other processing steps, optimizing gene expression and ensuring accurate mRNA maturation.

ATP-dependent chromatin remodeling complexes and histone chaperones disrupt histone-DNA interactions, restructuring or repositioning nucleosomes to create nucleosome-free regions. After pre-mRNA synthesis, these proteins rewind DNA around histones to reassemble nucleosomes, allowing transcription machinery continuous access to condensed genomic DNA throughout the process.

Transcription elongation follows transcription initiation and termination, during which initial transcription factors are released and RNA polymerase adds nucleotides to the growing RNA strand's 3' end. This phase is dynamic and varies based on DNA sequence heterogeneity, requiring multiple accessory proteins to ensure complete and accurate gene transcription.

The elongation complex's composition varies along the gene during transcription due to sequence heterogeneity of the DNA being transcribed. Different elongation factors and regulatory proteins associate with RNA polymerase at different stages, allowing the cell to respond to specific transcriptional challenges and regulate gene expression dynamically.