Prokaryotic_transcriptional_regulation

Transcriptional Regulation in Prokaryotes

Overview of Transcriptional Regulation

• Transcriptional regulation is crucial for controlling cellular states in prokaryotes.

• It determines the levels of mRNA transcribed from genes, directly affecting protein synthesis.

• Key players include:

  • RNA Polymerase: Binds to promoters to initiate transcription.

  • Activator Proteins: Enhance RNA polymerase binding to promote gene expression.

  • Repressor Proteins: Block RNA polymerase binding, suppressing transcription.

Sigma Factors

• Sigma Factor: A protein essential for RNA polymerase to recognize promoters in bacteria.

  • Sigma Factor 70: The most common sigma factor involved in general growth.

  • Recognizes specific sequences in the promoter regions known as the -10 (TATA box) and -35 regions.

Promoter Structure

• Promoters contain important sequences that guide transcription:

  • Transcription Start Site: Designated as +1, where transcription begins.

  • -10 Region: Located 10 nucleotides upstream; contains TATA box (e.g., ATTAAT).

  • -35 Region: Positioned 35 nucleotides upstream; contains sequences like ATTGAC.

  • UP Element: Additional AT-rich regions that can enhance transcription in certain genes.

Mechanism of Transcription Initiation

• The sigma factor binds to the promoter region, facilitating the formation of an open complex through:

  • Separation of the template and non-template strands of DNA, creating a transcription bubble.

  • RNA polymerase begins RNA synthesis from the template strand.

Transcription Termination

• Two main strategies for transcription termination in prokaryotes:

  • Rho-independent Termination: Formation of a hairpin structure in the mRNA, recognized by the protein NusA, which, combined with AU-rich sequences, triggers dissociation of RNA polymerase and mRNA.

  • Rho-dependent Termination: The Rho helicase binds to the rut site (CA-rich sequence) in RNA, moving along to terminate transcription by separating RNA from the DNA-RNA hybrid complex.

Operons in Prokaryotes

• Operons: Groups of genes that are transcribed together due to their related functions, predominantly seen in bacterial cells.

• Lac Operon: A classic example that regulates lactose metabolism:

  • Contains genes lacZ, lacY, and lacA for lactose breakdown.

  • Controlled by the lac repressor (encoded by lacI) that blocks transcription when lactose is absent.

  • Allolactose binds to the repressor, allowing transcription when lactose is present, particularly in low glucose conditions, facilitated by cyclic AMP (cAMP).

Induction and Repression of the Lac Operon

• Lactose present → Allolactose binds lac repressor → RNA polymerase can bind and transcribe.

• Glucose present → lac repressor binds operator, preventing transcription even in the presence of lactose.

• Low glucose + lactose → cAMP and CAP bind to promote RNA polymerase binding for efficient transcription.

Experimental Insights into Regulation

• Mutant analyses helped distinguish between cis-acting (DNA sequences) and trans-acting (proteins) regulatory elements of the lac operon.

  • Neurodiploids: Lab-created strains with two copies of lac operon genes to analyze interactions between cis and trans elements.

Arabinose Operon

• Arabinose: Another sugar regulated similarly, featuring the AraC protein, which can act as both an activator and a repressor depending on arabinose presence.

• When arabinose binds AraC, it allows RNA polymerase to bind and transcribe necessary genes for arabinose catabolism.

Tryptophan Operon

• Tryptophan Operon: A type of operon that is typically on but can be repressed when tryptophan levels are sufficient:

  • The TRP repressor protein is activated by tryptophan, preventing its own synthesis when levels are adequate.

  • Also features a leader sequence for further regulation through transcriptional attenuation, where high tryptophan levels lead to shorter mRNA and cessation of transcription.