5th edition 938-978

Chapter 26: Transcription and RNA Processing

RNA polymerase (RNAP) structure exhibits homology to DNA polymerase, indicating similarities in their enzymatic functions.
In prokaryotes, a single RNA polymerase enzyme is responsible for the synthesis of all RNA types, including mRNA, rRNA, and tRNA, except for the short RNA primers used during DNA replication.

Transcription is initiated at specific sequences of DNA known as promoter regions, which are typically located upstream of the genes they regulate.
The holoenzyme, which consists of RNA polymerase and a sigma (σ) factor, binds to the promoter, recognizing specific nucleotides in the promoter sequence.
Upon binding, the DNA strands undergo local unwinding, forming an open complex that exposes the template strand for transcription.

RNA polymerase reads the DNA template strand in the 3′ to 5′ direction, while synthesizing the RNA strand in the 5′ to 3′ direction, which allows for the addition of nucleotides to the 3′ end.
The growing RNA strand begins with a 5′-triphosphate group, providing a high-energy starting point for further nucleotides to be added.
Importantly, elongation proceeds without the need for a primer, unlike DNA replication, enabling a more streamlined transcription process.

Transcription terminates at specific sequences within the DNA known as terminators.
There are two main mechanisms for termination: intrinsic terminators can form a hairpin loop in the growing RNA followed by a U-rich sequence, prompting RNA polymerase to disengage from the DNA; while Rho-dependent termination involves the Rho factor binding to the RNA and traveling along it to catch up to RNA polymerase, facilitating termination.

Eukaryotic cells possess three distinct RNA polymerases, each responsible for synthesizing different types of RNA:
- RNA Polymerase I: Responsible for synthesizing ribosomal RNA (rRNA) precursors in the nucleolus, which are critical for ribosome formation.
- RNA Polymerase II: Synthesizes messenger RNA (mRNA) precursors in the nucleoplasm, playing a crucial role in gene expression and protein production.
- RNA Polymerase III: Synthesizes transfer RNA (tRNA) and small nucleolar RNAs (snoRNAs), which are essential for protein synthesis and rRNA processing, respectively.

Each eukaryotic RNA polymerase recognizes different types of promoters that exhibit varying levels of complexity. The core promoter, often containing motifs like the TATA box, is essential for the initiation of transcription.

General transcription factors (GTFs) are indispensable in the eukaryotic transcription initiation process as they facilitate the recruitment of RNA polymerase II to the promoter, resulting in the formation of an active preinitiation complex (PIC).
This complex undergoes a series of conformational changes, ultimately leading to the unwinding of DNA and the start of transcription.

Eukaryotic mRNAs undergo extensive posttranscriptional modifications to enhance stability and functionality:
- 5′ Capping: The addition of a 7-methylguanosine cap at the 5′ end protects the mRNA from degradation and promotes ribosome recognition during translation.
- 3′ Polyadenylation: A poly(A) tail, which consists of roughly 250 adenosine nucleotides, is added to the 3′ end to further protect the mRNA from degradation and assist in the regulation of its translation.

Pre-mRNA molecules often contain non-coding regions called introns intermixed with coding regions known as exons. The process of splicing removes these introns, facilitating the joining of exons to form mature mRNA.
Splicing is carried out by the spliceosome, a complex of proteins and small nuclear ribonucleoproteins (snRNPs) that orchestrates two transesterification reactions to excise introns effectively.

A significant feature of eukaryotic gene expression, alternative splicing allows the same gene to code for multiple protein isoforms by utilizing different combinations of splice sites, thus increasing protein diversity without requiring additional genes. This process plays a vital role in developmental regulation and cellular response to environmental cues.

RNA editing allows for posttranscriptional modification of nucleotides in the mRNA, altering the encoded protein's structure and function. An example includes the deamination of cytidine to uridine.
Small nucleolar RNAs (snoRNAs) play roles in guiding the methylation and processing of rRNA, essential for ribosome biogenesis.

rRNA is initially synthesized as a larger precursor molecule that undergoes a series of cleavage and modification steps, resulting in the generation of mature rRNA species necessary for ribosome assembly and function.

tRNA precursors are subject to extensive processing, including trimming of excess nucleotides and removal of introns. Following processing, a conserved 3′ CCA sequence is added post-transcriptionally, which is critical for tRNA's function in protein synthesis.

Central Dogma of Molecular Biology: This fundamental principle describes the flow of genetic information as being transferred from DNA to RNA and then to protein, a process central to gene expression.
Types of RNA: mRNA (messenger), rRNA (ribosomal), and tRNA (transfer) each serve distinct roles, working collaboratively to facilitate the expression of genetic information into functional proteins.
Regulation of Transcription: Various sigma factors in prokaryotes and transcription factors in eukaryotes serve to modulate gene expression in response to environmental stimuli and developmental signals.
RNA Processing Importance: Critical for producing mature RNA capable of efficient translation, the processing involves capping, tailing, splicing, and other modifications necessary for functional gene expression.