8. Molecular Biology of Transcription and RNA Processing

RNA Transcripts Carry the Messages of Genes

Understanding how genetic information in DNA directs protein synthesis was a major milestone in molecular genetics. RNA plays a central role as the intermediary between DNA and protein synthesis.

• Key Point 1: After the discovery of DNA structure, researchers investigated how DNA information is used to synthesize proteins.

• Key Point 2: RNA is abundant in all cells and chemically similar to DNA, but its function was initially unclear.

• Key Point 3: In eukaryotes, DNA is located in the nucleus, while protein synthesis occurs in the cytoplasm, suggesting RNA's role as a messenger.

RNA Structure and Types

RNA Nucleotides and Structure

RNA is composed of ribonucleotides, which are similar to DNA nucleotides but with two critical differences.

• Key Point 1: RNA nucleotides contain the sugar ribose instead of deoxyribose.

• Key Point 2: The base uracil (U) replaces thymine (T) found in DNA.

• Key Point 3: The other bases—adenine (A), guanine (G), and cytosine (C)—are the same as in DNA.

Example: Figure 8.1 shows the chemical structures of the four RNA ribonucleotides: AMP, GMP, UMP, and CMP.

RNA Assembly and Structure

RNA and DNA have similar sugar-phosphate backbones, and RNA is synthesized using DNA as a template.

• Key Point 1: RNA strands are assembled by forming phosphodiester bonds between adjacent nucleotides.

• Key Point 2: RNA synthesis uses complementary base pairing: A pairs with U, and C pairs with G.

Example: Figure 8.2 illustrates the process of RNA synthesis from a DNA template.

RNA Synthesis

RNA polymerase catalyzes the synthesis of RNA from a DNA template.

• Key Point 1: RNA polymerase adds ribonucleotides to the 3' end of the growing RNA strand, forming phosphodiester bonds.

• Key Point 2: Two phosphates are released during each addition, similar to DNA synthesis.

Equation:

Experimental Discovery of Messenger RNA (mRNA)

Pulse-Chase Experiments

Pulse-chase experiments were crucial in tracing the synthesis and fate of RNA in cells.

• Key Point 1: Cells are exposed to radioactive nucleotides (pulse), then to nonradioactive nucleotides (chase).

• Key Point 2: The movement of radiolabeled RNA is tracked to determine its synthesis and degradation.

• Example: Volkin and Astrachan (1957) showed that bacteriophage infection in bacteria triggers a burst of RNA synthesis, necessary for new phage protein formation.

Experiments in Eukaryotes

• Key Point 1: Radioactive uracil initially accumulates in the nucleus, then moves to the cytoplasm, supporting the role of RNA as a messenger from nucleus to cytoplasm.

mRNA, the Genetic Messenger

• Key Point 1: Brenner, Jacob, and Meselson identified an unstable RNA form (mRNA) that directs protein synthesis in E. coli infected with bacteriophage T2.

• Key Point 2: mRNA is rapidly synthesized and degraded, acting as a transient genetic messenger.

Categories and Functions of RNA

Messenger RNA (mRNA)

• Key Point 1: mRNA is produced by protein-coding genes and serves as a short-lived intermediary between DNA and protein.

• Key Point 2: mRNA is the only RNA type that undergoes translation.

• Key Point 3: mRNA often undergoes posttranscriptional processing in eukaryotes.

Functional RNAs

• Key Point 1: Ribosomal RNA (rRNA): Combines with proteins to form ribosomes, the site of protein synthesis.

• Key Point 2: Transfer RNA (tRNA): Binds specific amino acids and delivers them to the ribosome during translation.

Additional Functional RNAs

• Key Point 1: Telomerase RNA: Provides a template for telomere DNA synthesis.

• Key Point 2: Small nuclear RNA (snRNA): Involved in mRNA processing (e.g., splicing) in the nucleus of eukaryotes.

• Key Point 3: Micro RNA (miRNA) and small interfering RNA (siRNA): Regulate gene expression posttranscriptionally in plants and animals.

Table: Major RNA Molecules

Bacterial Transcription

Overview of Bacterial Transcription

Transcription in bacteria is the process of synthesizing a single-stranded RNA molecule from a DNA template, catalyzed by RNA polymerase. It occurs in four stages:

1. Promoter recognition

2. Transcription initiation

3. Chain elongation

4. Chain termination

DNA Strand Identification for Transcription

• Key Point 1: The template strand of DNA is used by RNA polymerase to assemble a complementary, antiparallel RNA strand.

• Key Point 2: The coding strand (nontemplate strand) is complementary to the template strand and has the same sequence as the RNA (except T is replaced by U).

Gene Structure

• Key Point 1: The promoter is immediately upstream (5') of the transcription start site (+1 nucleotide) and controls RNA polymerase access.

• Key Point 2: The coding region contains the information for protein synthesis.

• Key Point 3: The termination region is downstream (3') of the coding region and signals the end of transcription.

Example: Figure 8.3 shows the arrangement of promoter, coding region, and termination region in a gene.

Bacterial RNA Polymerase

• Key Point 1: A single RNA polymerase synthesizes all types of RNA in E. coli.

• Key Point 2: The antibiotic rifampicin inhibits RNA synthesis by blocking the first phosphodiester bond formation.

RNA Polymerase Composition

• Key Point 1: The core enzyme consists of five subunits: two α, two β, and one ω.

• Key Point 2: The sigma (σ) subunit binds to the core enzyme to form the holoenzyme, which is required for promoter recognition and initiation.

• Key Point 3: Alternative sigma subunits allow the holoenzyme to recognize different promoter sequences.

Example: Figure 8.4 illustrates the assembly of the RNA polymerase holoenzyme.

Bacterial Promoters and Consensus Sequences

• Key Point 1: Promoters are double-stranded DNA sequences that serve as binding sites for RNA polymerase and other transcription proteins.

• Key Point 2: Promoters contain consensus sequences at specific positions:

  • -10 (Pribnow box): 5'-TATAAT-3'

  • -35 region: 5'-TTGACA-3'

• Key Point 3: RNA polymerase binds to these sequences and initiates transcription at the +1 site.

Example: Figure 8.5 shows the structure of a bacterial promoter and consensus sequences.

Transcription Initiation

• Key Point 1: Initiation involves two steps: loose binding to the promoter (closed complex), followed by DNA unwinding (open complex).

• Key Point 2: The holoenzyme unwinds about 18 base pairs of DNA around the -10 position.

• Key Point 3: Sequence variation among promoters allows for regulation by different sigma subunits.

Example: Figure 8.6 (Parts 1-2) depicts the formation of closed and open promoter complexes.

Table: Escherichia coli RNA Polymerase Sigma Subunits

Transcription Elongation and Termination

• Key Point 1: After initiation, the sigma subunit dissociates, and the core enzyme continues RNA synthesis.

• Key Point 2: The DNA double helix is unwound ahead of the enzyme and reforms behind it.

• Key Point 3: When the gene is fully transcribed, the RNA transcript is released, and the core enzyme dissociates from DNA.

• Key Point 4: Multiple rounds of transcription can occur in succession.

Example: Figure 8.6 (Parts 3-5) shows elongation and termination steps in bacterial transcription.