Transcriptional Regulation and Prokaryotic Gene Expression

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Questions and Answers

What primarily characterizes prokaryotic genes in terms of expression?

They require multiple initiators for transcription.

They are mostly constitutively expressed. (correct)

They do not utilize RNA polymerase.

They are regulated by complex mechanisms.

What is a polycistronic mRNA?

A transcript that cannot be translated.

A transcript encoding a single protein.

A transcript encoding multiple proteins. (correct)

A transcript with no leader sequence.

In prokaryotic gene regulation, what role do repressor proteins play?

They activate transcription directly.

They inhibit transcription. (correct)

They enhance RNA Polymerase binding.

They modify the promoter structure.

What is an operon?

An expression unit consisting of related genes. Signup and view all the answers

What do the -10 and -35 elements in a promoter refer to?

They are consensus sequences essential for RNA polymerase binding. Signup and view all the answers

What does negative regulation of transcription involve?

Inhibition through repressor proteins. Signup and view all the answers

Which of the following describes a regulon?

A group of operons regulated together. Signup and view all the answers

What is indicated by the term 'regulatory site' in the context of transcription?

Regions where transcriptional activators or repressors bind. Signup and view all the answers

What is the primary function of an activator protein in transcription?

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

What role do architectural regulators serve in gene expression?

They bend DNA to promote looping. Signup and view all the answers

How do transcription coactivators function in gene regulation?

They bridge the RNA polymerase with activators. Signup and view all the answers

What is the function of insulators in gene expression?

They block the unintended effects of DNA looping. Signup and view all the answers

What is a characteristic feature of the bacterial Lac repressor?

It is composed of four identical subunits. Signup and view all the answers

In which way do transcription corepressors differ from activators?

Corepressors suppress transcription by binding activators. Signup and view all the answers

What is the main role of DNA looping in transcriptional regulation?

To enable recruitment of RNA polymerase by a promoter. Signup and view all the answers

What type of protein is CTCF, and what is its significance?

A highly conserved zinc finger protein acting as an insulator. Signup and view all the answers

What is the primary role of an effector in gene transcription?

It binds to activators or repressors, causing conformational changes. Signup and view all the answers

How can an effector inactivate a repressor?

By causing the repressor to dissociate from DNA. Signup and view all the answers

Which of the following statements about effectors is incorrect?

Effectors are typically large proteins. Signup and view all the answers

What happens when an effector is present and binds to an activator?

The activator binds to DNA and stimulates transcription. Signup and view all the answers

Coordinated regulation of gene expression typically occurs in response to what?

Stress responses. Signup and view all the answers

Which mechanism can lead to the global regulation of multiple genes?

Removals of a common repressor by an effector. Signup and view all the answers

What effect does the absence of an effector have on activators?

Activators bind DNA and facilitate transcription. Signup and view all the answers

What is NOT a way that effectors can regulate transcription?

Binding directly to RNA. Signup and view all the answers

Study Notes

Transcriptional Regulation of Gene Expression

Regulatory DNA-binding proteins control gene expression
Text sections covered: 19.1-19.2, 20.1 (SOS response)
Key terms: activation, repression, activator, repressor, regulatory site, positive regulation, negative regulation, DNA looping, coactivator, corepressor, insulator, effector, polycistronic mRNA, operon, regulon

Prokaryotic Gene Expression

Prokaryotic DNA is less condensed than eukaryotic DNA
Prokaryotic genes are often constitutively expressed
RNA polymerase generally has access to all promoters
Promoters have -10 and -35 elements
- -10 element (TATA box/Pribnow box) consensus sequence: TATAAT/A
- -35 element consensus sequence: TTGACA
Regulatory molecules affect RNA polymerase binding to the promoter
- Operator: repressor binding site
- Activator binding site:Activator binding site
Genes transcribed as a unit

Polycistronic mRNA

A transcript encoding multiple proteins
Includes a leader sequence preceding the first gene
Subsequent genes are followed by intercistronic regions

Operon

An expression unit consisting of one or more co-transcribed genes and their regulatory sequences

Regulon

A group of genes or operons that are coordinately regulated
Genes may be spread throughout/distant in the chromosome/genome

Negative Regulation

Repressor proteins inhibit transcription
Repressor-binding site overlaps the promoter
Repressor binding prevents RNA polymerase from initiating transcription

Positive Regulation

Some genes need activators to bind to their promoters to initiate transcription
Activators recruit RNA polymerase to the promoter
Transcription then initiates

Transcriptional Regulation From a Distance: DNA Looping

Some transcription activators bind to a distant regulatory site
Enabling the promoter and RNA polymerase complex to form a DNA loop

DNA Looping Mediation

Single regulatory proteins (e.g., Lac repressor) bind to two distant sites
Forming a DNA loop

Architectural Role of Regulatory Proteins

Architectural regulators bend DNA
Promoting DNA looping

Transcription Coactivator

A protein that facilitates transcription
"Bridges" RNA polymerase with transcription activators
Does not directly bind DNA. (e.g., Mediator complex)

Transcription Corepressor

A protein that binds to an activator to suppress transcription
Does not bind RNA polymerase
(e.g., Yeast Cys8)

Insulators

Block unintended effects of distant DNA looping
Present in both prokaryotes and eukaryotes
Prevents cross-talk between regulators of different promoters
- CTCF (CCCTC-binding factor) is an example of a highly conserved zinc finger protein, acting as an insulator

Effectors

Small molecules (not proteins)
Bind to activators or repressors
Induce a conformational change
Results in increased/decreased transcription of a target gene

SOS Response (global regulation)

Mechanism for repairing damaged DNA in bacteria
Expression of multiple genes involved in repair is coordinated
Requires two key proteins: RecA and LexA
- LexA normally represses SOS genes
- RecA protein (activated if DNA is damaged) triggers LexA autocleavage, removing repression, allowing transcription of SOS genes

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Description

Explore the intricacies of transcriptional regulation and prokaryotic gene expression in this quiz. Understand the roles of regulatory proteins, DNA elements, and polycistronic mRNA. Key concepts include activation, repression, and the importance of promoter elements in prokaryotes.