Transcription

Structural organization of the template

RNA synthesis occurs at strictly defined regions of the genome. The unit of transcription is the transcription unit, a DNA fragment bounded by two key regulatory elements:

• Promoter — a specific nucleotide sequence (site) where RNA polymerase binds to the template and initiates assembly.
• Termination site — the region where polynucleotide chain synthesis concludes.

The main feature of eukaryotes is that a single transcription unit typically includes only one gene. This isolated arrangement allows the cell to individually read different genes, finely regulating their activity according to its needs.

Eukaryotic RNA polymerases

In the nucleus of a eukaryotic cell, biosynthesis is catalyzed by massive multisubunit enzymes — RNA polymerases. They not only form phosphodiester bonds but also possess binding centers for regulatory factors. Depending on the type of synthesized product, three types of enzymes are distinguished:

1. RNA polymerase I — specializes in the synthesis of ribosomal RNA precursors (pre-rRNA).
2. RNA polymerase II — responsible for generating messenger RNA precursors (pre-mRNA).
3. RNA polymerase III — catalyzes the assembly of transfer RNA precursors (pre-tRNA).

The construction of the new chain proceeds according to the principle of complementarity. Opposite DNA adenine (A), RNA uracil (U) is incorporated; opposite guanine (G), cytosine (C); and opposite thymine (T), adenine (A).

Initiation stage

The process is initiated in the promoter region, where a crucial role is played by a protein — the TATA-binding factor. It recognizes a specific nucleotide sequence ("-TATA-") and binds to it.

The attachment of the TATA-factor "activates" the promoter, facilitating its interaction with RNA polymerase. The docking of the enzymatic complex onto the DNA sharply increases its affinity for initiation factors (A, B). Through joint efforts, they ensure the local unwinding of approximately one turn of the DNA double helix to gain direct access to the template strand for the start of polymerization.

Elongation and termination

Following successful initiation, elongation begins — the active growth of the pre-RNA strand. Synthesis of the molecule always proceeds antiparallel, in the 5'- to 3'-direction, while the enzyme itself moves along the template from the 3'- to the 5'-end. At this stage, elongation factors (E, H, F) are recruited, which enhance enzyme activity and maintain the local separation of the strands.

The dynamics of the DNA helix are such that ahead of the sliding polymerase, the turns unwind, while behind it, they re-anneal. The overall reaction equation is as follows:

aGTP + bATP + cUTP + mCTP → (DNA template, Mg2+, RNA polymerase) → RNA + (a+b+c+m)H4P2O7

The process concludes at the termination stage. When the complex reaches the termination site, the local unwinding of the strands makes this region accessible to termination factors. They halt the polymerase's activity, accelerating the detachment of the primary transcript and the release of the enzyme from the complex with DNA.

Briefly: answers to topical questions

What else to study on this topic

• Post-transcriptional modifications of primary transcripts
• Functions of initiation factors A and B
• Mechanism of action of elongation factors E, H, F
• The process of pyrophosphate cleavage during chain elongation

Other topics in the section "Template Biosyntheses"

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