Gene Regulation and Expression

Gene Regulation

• Cells can change the expression of genes, as seen in the nucleus of an adult skin cell being transferred to an enucleated egg, resulting in an entire tadpole.

Constitutive Genes

• Constitutive genes are essential components of almost all living cells, performing "housekeeping" functions, such as molecules involved in protein synthesis (rRNA, tRNA, RNA polymerase subunits, etc.).
• These genes are continuously expressed in most cells.

E. Coli RNA Polymerase

• The core enzyme consists of α, ββ'ω subunits, while the holoenzyme consists of α2ββ'ωσ subunits.
• Functions of the subunits include:
  • α: assembly of the tetrameric core
  • β: ribonucleoside triphosphate binding site
  • β': DNA template binding region
  • σ: initiation of transcription
• Regulation at transcription initiation involves altering the specificity of RNA Polymerase-promoter interaction, impeding access of RNA Polymerase to its promoter, or using activators to enhance polymerase-promoter activity.

Inducible and Repressible Genes

• Inducible genes are expressed when needed, while repressible genes are turned off when not needed.
• Examples include the synthesis of alcohol dehydrogenase, which is only expressed when ethanol is present.

DNA Sequences Involved in Regulation of Gene Expression

• Most bacterial promoters include the conserved -10 region (Pribnow box) and -35 region, which interact with the σ factor of RNA polymerase.
• Some promoters also include the upstream element, which interacts with the α subunit of RNA polymerase.

Mechanisms to Regulate Transcription in Bacteria

• Two main mechanisms to regulate transcription in bacteria are:
  1. Using different σ factors in RNA Polymerase, such as sigma70 and sigma32, which recognize different classes of promoters and allow for coordinated expression of different sets of genes.
  2. Binding other proteins, transcription factors, to promoters, which recognize promoters of specific genes, bind small signaling molecules, undergo post-translational modifications, and alter their affinity toward DNA.

Negative Regulation

• In negative regulation, a bound repressor inhibits transcription by binding to the operator region of DNA, preventing RNA polymerase from binding to the promoter region.
• The repressor protein can be released from the operator by a molecular signal, allowing transcription to occur.

Positive Regulation

• In positive regulation, a bound activator facilitates transcription by binding to the promoter region of DNA, enhancing the binding of RNA polymerase to the promoter.
• The activator protein can be released from the promoter by a molecular signal, preventing transcription.

The Lac Operon

• The lac operon is a classic example of gene regulation, consisting of three linked structural genes (lacz, lacy, and lacA) that are transcribed as a unit.
• The lac operon has a promoter, operator, and activator binding site, and is regulated by the lac repressor and CRP (catabolite activator protein).

Induction of the Lac Operon

• The lac operon is induced when lactose is present and glucose is absent, allowing the cell to utilize lactose as an energy source.
• The inducer allolactose binds to the repressor, causing it to release from the operator, and allowing transcription of the structural genes to occur.

The Operon Model

• The operon model is a classic model of gene regulation, consisting of a cluster of related genes that are turned off or on together.
• It includes binding sites for activators and repressors, a promoter, and one or more genes whose expression is controlled by the operon.