Bacterial Transcription and RNA Synthesis

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What initiates the elongation phase of RNA synthesis?

The σ subunit begins to synthesize DNA

The formation of the first phosphodiester bond

The transcript exceeding ten nucleotides in length (correct)

The binding of the ρ factor to the RNA polymerase

Which aspect of RNA polymerase is true during elongation?

It unwinds the DNA helix as it synthesizes RNA (correct)

It has proofreading activity to correct errors

It releases ribonucleoside triphosphates during synthesis

It requires a primer for synthesis

What is required for ρ-dependent termination of RNA synthesis?

Binding of RNA polymerase to the promoter region

Involvement of the Rho (ρ) factor (correct)

Release of pyrophosphate from ribonucleoside triphosphates

Action of the DNA topoisomerases

How does Rifampin affect bacterial transcription?

It inhibits transcription initiation by binding to the β-subunit of RNA polymerase Signup and view all the answers

What characterizes an operon in bacterial transcription?

Grouped structural genes with regulatory genes on the chromosome Signup and view all the answers

What is the primary function of the lacZ gene in the lac operon?

Codes for β-galactosidase to hydrolyze lactose Signup and view all the answers

What happens when both glucose and lactose are present in the environment?

Transcription of the lac operon is effectively blocked despite the absence of the repressor. Signup and view all the answers

How does allolactose affect the repressor protein in the lac operon?

It prevents the repressor from binding to the operator. Signup and view all the answers

What role does the CAP (cAMP regulatory protein) play in the regulation of the lac operon?

It activates RNA polymerase binding to the promoter. Signup and view all the answers

What is the physiological function of the enzyme coded by the lacA gene?

Its physiological function is unknown. Signup and view all the answers

What is the primary role of the sigma factor in prokaryotic RNA polymerase?

To recognize promoter regions on the DNA Signup and view all the answers

Which region of DNA is recognized by the RNA polymerase during transcription initiation?

Promoter region Signup and view all the answers

What is the function of the core enzyme in prokaryotic RNA polymerase?

To facilitate RNA synthesis Signup and view all the answers

Where is the -35 consensus sequence located in relation to the transcription start site?

35 bases to the left Signup and view all the answers

What happens during the elongation phase of transcription in prokaryotes?

RNA is synthesized from the DNA template Signup and view all the answers

Which of the following best describes the holoenzyme in prokaryotic RNA polymerase?

Sigma factor and core enzyme combined Signup and view all the answers

What is the role of the rho (ρ) factor in prokaryotic transcription?

Recognizes termination sequences Signup and view all the answers

What outcome can a mutation in the -10 or -35 sequence have on transcription?

It can inhibit transcription Signup and view all the answers

What is the role of ribosomal RNA (rRNA) in the cell?

It is the most abundant type of RNA in the cell. Signup and view all the answers

Which characteristic best describes transfer RNA (tRNA)?

It carries a specific amino acid to the site of protein synthesis. Signup and view all the answers

What distinguishes eukaryotic mRNA from prokaryotic mRNA?

Eukaryotic mRNA has a poly-A tail and a 5’ cap. Signup and view all the answers

What is the primary function of messenger RNA (mRNA) in protein synthesis?

It carries genetic information from DNA to the ribosomes. Signup and view all the answers

Which of the following is true regarding the sizes of RNA types in a cell?

rRNA makes up approximately 80% of total RNA. Signup and view all the answers

What is polycistronic mRNA and where is it commonly found?

It refers to mRNA that codes for multiple proteins, typical in prokaryotes. Signup and view all the answers

Which RNA type is essential for recognizing codons during translation?

Transfer RNA (tRNA) Signup and view all the answers

What is the significance of the 5’ cap and poly-A tail in eukaryotic mRNA?

They are involved in translation initiation and stability. Signup and view all the answers

Study Notes

RNA Structure & Synthesis (I)

RNA has three main types crucial for protein synthesis: ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA).
Ribosomal RNA (rRNA) is associated with several proteins in ribosomes, serving as protein synthesis sites. Prokaryotic rRNA includes 23S, 16S, and 5S; eukaryotic rRNA includes 28S, 18S, 5.8S, and 5S. The S values (Svedberg units) relate to molecular weight and shape. rRNA accounts for approximately 80% of total RNA in a cell.
Transfer RNA (tRNA) is the smallest RNA molecule (4S), with at least one specific type for each of the 20 amino acids commonly found in proteins. It comprises about 15% of total RNA. tRNA acts as an adaptor, carrying its specific amino acid covalently attached to its 3' end to protein synthesis sites. It recognizes the genetic code word on mRNA, determining which amino acid is added to the growing peptide chain.
Messenger RNA (mRNA) makes up only about 5% of RNA. It is the most heterogeneous RNA type, varying in size (500 to 6000 nucleotides) and base sequence. mRNA carries genetic information from nuclear DNA to the cytosol. mRNA can be polycistronic (carrying information from multiple genes, characteristic of prokaryotes) or monocistronic (carrying information from a single gene, characteristic of eukaryotes). Eukaryotic mRNA has a poly-A tail (long sequence of adenine nucleotides) at the 3' end and a "cap" (molecule of 7-methylguanosine) at the 5' end.

Prokaryotic and Eukaryotic rRNAs

Prokaryotes have 16S, 23S, and 5S rRNA.
Eukaryotes have 18S, 5.8S, 28S, and 5S rRNA.

Prokaryotic RNA Polymerase

RNA polymerase in bacteria synthesizes all RNA except for short RNA primers required for DNA replication.
It's a multisubunit enzyme recognizing a nucleotide sequence (the promoter region) at the beginning of a transcribed DNA segment.
It creates a complementary RNA copy from the DNA template strand.
It recognizes the end (terminal region) of the DNA sequence to be transcribed.

Core Enzyme

The core enzyme is required for enzyme assembly.
It's responsible for template binding and 5'→3' RNA polymerase activity.
Critically, it cannot recognize the promoter region on DNA.

Holoenzyme

The addition of the σ subunit (sigma factor) creates the holoenzyme.
This enables RNA polymerase to recognize promoter regions on DNA.
Different σ factors recognize different gene groups.

Termination Factor

Some DNA regions signal transcription termination directly recognized by the RNA polymerase itself.
Other termination sequences are recognized by specific termination factors (like rho (ρ) factor in E. coli).

Steps in RNA Synthesis (Prokaryotes)

RNA synthesis occurs in three phases: initiation, elongation, and termination.
Initiation: RNA polymerase holoenzyme binds to the promoter region. Prokaryotic promoters have characteristic consensus sequences like -35 and Pribnow (or -10) box. The -35 sequence (5'-TTGACA-3') is centered about 35 nucleotides from the transcription start point and the Pribnow box (5'-TATAAT-3') is centered about -10.
Elongation : Holoenzyme recognizes the promoter region and starts synthesizing DNA sequence (often a purine) transcript. Elongation phase starts when the transcript exceeds ten nucleotides. The σ subunit is released during elongation.

Elongation (cont'd...)

RNA polymerase doesn't use a primer.
It doesn't have proofreading activity.
It uses ribonucleoside triphosphates and releases pyrophosphates during nucleotide addition to growing chains.
Binding of RNA polymerase to the DNA template causes a local unwinding of the DNA helix, leading to the formation of supercoils that can be relaxed by DNA topoisomerase.

Termination

The RNA chain elongation process continues until a termination signal is reached.
Depending on the signal, either p-dependent termination (requiring an additional Rho (ρ) factor) or p-independent termination (no additional factor needed). P-independent termination is characterized by the presence of a hairpin loop in the nascent RNA, which helps halt transcription.

Action of Antibiotics

Some antibiotics inhibit bacterial RNA synthesis, and this can prevent bacterial growth.
Rifampin is an example, binding to the bacterial RNA polymerase subunit (β), hindering transcription initiation and the first phosphodiester bond formation.

Transcription from bacterial operons

In bacteria, structural genes for metabolic pathway enzymes often group on chromosomes along with regulatory genes, forming transcription units called operons.
These genes are coordinately expressed as a long mRNA molecule. The lactose operon in E. coli is a key example.

The Lactose (lac) Operon

The lac operon controls three enzymes involved in lactose catabolism: β-galactosidase (lacZ), permease (lacY), and thiogalactoside transacetylase (lacA).
The regulatory portion of the operon involves a catabolite activator protein (CAP or CRP) binding site, the promoter, and the operator site.

Lactose Operon (conditions)

Off (with glucose): The repressor protein binds to the operator site, blocking RNA polymerase. No mRNA, and therefore, no proteins are produced.
On (with lactose): Lactose (converted to allolactose) binds to the repressor protein, causing a conformation change that prevents it from binding to the operator site. RNA polymerase can initiate transcription to produce mRNA, which is then translated into proteins necessary for lactose metabolism
Operon with glucose and lactose (off): When both glucose and lactose are present, adenyl cyclase is deactivated, preventing cAMP production. A cAMP-CAP complex can't form, and the CAP binding site remains empty. Even though the repressor is unbound, RNA polymerase still cannot effectively initiate transcription, so the lactose genes are not expressed.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz explores key concepts related to bacterial transcription and RNA synthesis. It covers topics such as the elongation phase of RNA synthesis, the roles of operons and specific genes like lacZ, and the effects of regulators like CAP and Rifampin. Test your knowledge on the mechanisms and factors involved in prokaryotic transcription.